A cubic-bezier curve in this context describes the movement of an element from a start point to an end point.
If the curve is just a straight line, the element moves from start to end with a constant speed.
Examples: cubic-bezier(0, 0, 1, 1) or cubic-bezier(0.2, 0.2, 0.44, 0.44)
(those two are identical in terms of the manner of the movement)
A steeper curve means that the element accelerates. A shallower curve makes the element slow down.
The four numbers that you enter are just parameters that define the shape of the curve.