If you are familiar with lerp() in GameMaker (linear interpolation) it’s like that, but rather than changing at a constant rate it changes with either an acceleration or deceleration. This video by Code Workshop explains easing far better than me.
However it doesn’t just have to be with movement, it could be with the angle or transparency etc. I’ve never found an easy way to explain what Easing and Tweening is but it’s basically gradually changing the speed something moves at rather than instantly changing its speed. None of the images above have animated sprites, they are all static images that are moved with easing.
It could be for making items smoothly move onto the screen, make things bounce around in a natural way, highlighting buttons when they are selected etc etc.
Easing algorithms are something I know I will need in every single game I make.