What's in a druid's grove?

Masters of the Wild web enhancement - Druids Grove (3.5e), which was a pdf addon to the corebook Masters of the Wild, contains the following info:

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• First, there must be a place for the druid to sleep. This can be a natural shelter, such as a cave, or one built from natural materials, such as a sod hut or tree house.
• Next, there must be a good supply of food and clean, fresh water. The food supply can include nuts and berries, tree fruit, and a diverse population of animals.
• In addition, most druids maintain at least one herb garden, and many also cultivate a patch of vegetables.
• The next vital feature is a system of protection for the druid and her charges, both plant and animal. This often takes the form of natural barriers, such as briar hedges or dense forest.
• Warmth is another vital issue. If the climate where the druid lives is temperate or cooler, she requires a good source of firewood, coal, or other fuel. In addition, druids use natural materials to make warm clothing and blankets. Each generally has a cache of tools stashed away for sewing hides and weaving cloth from plant fibers or animal hair.
• Druids like to be prepared for emergencies, so most keep a supply of important magical and nonmagical items secreted about their groves—in hollow trees, under stones, and in unused animal lairs. Stashed magical items usually include goodberries and infusions of useful spells (see Masters of the Wild, Chapter 3). Other useful items include small amounts of money (in case they need to visit a town), tools for starting a fire, knife and extra weapons.
• A typical druidic grove features some means for scrying—most often a pool of still, mirrorlike water. A large, highly polished rock also might make a good scrying device. In a pinch, a druid can use the create water spell to produce still water for scrying in a large container or natural crevice.

Those are the main points, slightly edited for brevity. The rest of the pdf is devoted to outlining a sample druid's grove for a 12th level Half Elf Druid named Thyme. The pdf is freely available for download at the link above.

In addition, if you have access to the 2e rulebook O Manual Completo do Druida, there are details on lesser and greater magical groves, which would add the benefits you are seeking, among which are the following (Lesser Grove):

• Druids entering the grove feel a watching presence and a sense of power. For every three rounds they spend within its boundaries they learn one power of the sacred grove, through a vision or intuition.
• All druids receive a +1 bonus to saving throws vs. spell, death magic, and wands while within a lesser magical grove. The grove’s steward receives a +2 bonus. All in the grove are rendered immune to magical fear while within it.
• Dig spells never work within a grove.
• Natural (nonmagical) lightning never strikes trees or beings in the grove.
• Evil enchanted creatures cannot enter the grove unless it has been defiled. (See ”Defiled and Cursed Groves” in this chapter).

There are tables to determine random benefits for lesser and greater magical groves (Of which not all groves are magical to begin with.)

This is not lore, but is instead focused on D&D 5E mechanics.

In Xanathar's Guide to Everything, there is a Spell called Druid Grove which magically creates a warded area--and if you repeat the spell in the same location for a year, the magic becomes permanent (until dispelled).

As for size

The area can be as small as a 30-foot cube or as large as a 90-foot cube.

As for what the warded area does...

The spell creates the following effects within the area. When you cast this spell, you can specify creatures as friends who are immune to the effects. You can also specify a password that, when spoken aloud, makes the speaker immune to these effects.

You get the following protections over the area...

Solid Fog. You can fill any number of 5-foot squares on the ground with thick fog, making them heavily obscured. The fog reaches 10 feet high. In addition, every foot of movement through the fog costs 2 extra feet. To a creature immune to this effect, the fog obscures nothing and looks like soft mist, with motes of green light floating in the air.

Grasping Undergrowth. You can fill any number of 5-foot squares on the ground that aren’t filled with fog with grasping weeds and vines, as if they were affected by an entangle spell. To a creature immune to this effect, the weeds and vines feel soft and reshape themselves to serve as temporary seats or beds.

Grove Guardians. You can animate up to four trees in the area, causing them to uproot themselves from the ground. These trees have the same statistics as an awakened tree, which appears in the Monster Manual, except they can’t speak, and their bark is covered with druidic symbols. If any creature not immune to this effect enters the warded area, the grove guardians fight until they have driven off or slain the intruders. The grove guardians also obey your spoken commands (no action required by you) that you issue while in the area. If you don’t give them commands and no intruders are present, the grove guardians do nothing. The grove guardians can’t leave the warded area. When the spell ends, the magic animating them disappears, and the trees take root again if possible.

Additional Spell Effect. You can place your choice of one of the following magical effects within the warded area:

• A constant gust of wind in two locations of your choice
• Spike growth in one location of your choice
• Wind wall in two locations of your choice

To a creature immune to this effect, the winds are a fragrant, gentle breeze, and the area of spike growth is harmless.

So, in short, it would be easy to make the assumption that mapped Druid Groves are simply places where a powerful Druids has made a Druid Grove spell permanent. Anything else that may be within them is up to the druids who live near there and use the Grove.

If you'd like further info on Druidism in Greyhawk, and of the spiritual significance of a Grove...

Sacred Groves: The sacred groves act as centers of worship for the Old Faith. They are the lesser of the two worship sites and are much more common than megalithic circles. Oak groves are the most prevalent, but other trees are used, such as the massive deklo or the fruit-bearing usk. The choice of trees in a grove is crucial as every tree has symbolism to the Old Faith.

As taken from an old D&D 3.5E article on WotC's website.

Note that as I am not Anna B Meyer, I cannot say for absolute certain that this was their intent with the groves placed on their maps.