Easing
Easing is a way to adjust an animation’s fraction. Easing allows transitioning elements to speed up and slow down, rather than moving at a constant rate.
Fraction is a value between 0 and 1.0 indicating our current point in the animation where 0 represents the start and 1.0 represents the end.
An Easing must map fraction=0.0 to 0.0 and fraction=1.0 to 1.0.