Interaction design is about how you users interact with your product. Its goal is to make this interaction smooth and intuitive.
Animations can make specific interactions easier to understood.
Interaction became a big thing when smart phones came into the picture. There are a huge variety of interactions you can apply to your design: double tap, pinch, swipe, 3-finger swipe, 3D touch and so on. This opened a new horizon in interaction design.
By now, we have well established interaction design patterns: when you pull down your e-mail client on your phone to update. This “pull down to refresh” became quite well-spread and most of the people become familiar with them.