Mouse acceleration is when the movement is based on speed. Which means if you move your mouse 1 inch slowly it will move your crosshair one distance and if you move it quickly the same distance your crosshair will travel further.

Usually it's better for gaming to turn this feature off since we usually rely on muscle memory for aiming and if we don't know where our crosshair will land, even if we move our mouse the same distance it can be difficult to develop a muscle memory.

Thanks!