Boeing bulletin issued last November said
"MCAS provides automatic nose-down inputs to assist pilots in certain manual, flaps-up flying scenarios, especially at slow airspeeds and high AOA."
John K provided a complete and detailed answer. Thank you. Since altitude is not significant, I have amended the question to be more appropriate for this site.
MCAS is basically an artificial stability system for the flaps-up low-speed regime, autopilot off, that is what I would call clever a band-aid solution to a pitch instability problem that showed up in development.
If you are hand flying at low speed and are trimmed at, say, 190 kt flaps up, and the airplane is on trim speed, the nose should be rock solid and not wander up or down on its own unless speed (or thrust) changes occur. If you are going 190kt and decelerate to 180, the nose should pitch down and vice versa if you accelerate (thrust being constant - thrust changes also change trim speed because it applies leverage to pitch the nose up, but let's not complicate things).
With the Max in that sort of configuration, hand flying, slow, flaps up, the pitch behavior didn't meet cert requirements and the nose would typically drift up on its own, which, if thrust is the same, raises AOA and makes the airplane slow down when it should be holding a constant pitch attitude and speed. This makes the airplane a handful to hand fly at low speed because the pilot has to actively make inputs to stop the drift in pitch, which really raises the workload (fly any other airplane with the C of G too far aft and you get something similar).
Soooo, they decided to use computers to run the horizontal stab in the background, transparent to the crew, to do the job of stopping the drift in pitch attitude and make the airplane behave the way it naturally should - an artificial stability system, but whose role was limited to fixing this particular characteristic in this particular configuration.
The alternative would probably have involved making the horizontal tail larger, which would have resulted in the usual cascade of side effects and really upset the apple cart of the development business case for the Max. A classic case of a software based fix for a thorny aerodynamic problem that avoids dealing with the actual aerodynamic problem because dealing with the actual problem might have blown up the program during development.
From what I understand it's related to the new engines, which have more power and hang lower and whose thrust line has more pitch up leverage as a result, and which reduces the down force required of the horizontal tail, similar to moving the CofG aft, among other factors (it's the tail's downforce balancing the center of gravity forward of the center of lift, an aerial see-saw you might say, that is where pitch stability comes from).