Slots de Feitiços Expandidos

O que exatamente são Slots de Feitiços Expandidos? Eles afetam feitiços conhecidos? Sou muito novo no D & D e não tenho certeza do que isso significa. Eu olhei no PHB, e achei isso:

The Warlock table shows how many spell slots you have. The table also shows what the level of those slots is; all of your spell slots are the same level. To cast one of your warlock spells of 1st level or higher, you must expend a spell slot. You regain all expended spell slots when you finish a short or long rest.

For example, when you are 5th level, you have two 3rd-level spell slots. To cast the 1st-level spell thunderwave, you must spend one of those slots, and you cast it as a 3rd-level spell.