In the Curve editor you may edit and save curves for use to further modify movements of objects.
The Curve Editor can be invoked wherever you select a curve, for example from the Movements dialog.
A curve is a number of points positioned in a coordinate system, along with a specification of the progression leading up to the point.
The X axis in a curve represents percentage of time whereas the Y axis represents a positional value in percent. Time in this sense is the length of the movement, while the positional value may well be a movement's extension X-wise.