Definition and Usage
The animation-timing-function specifies the speed curve of an animation.
The speed curve defines the TIME an animation uses to change from one set of CSS styles to another.
The speed curve is used to make the changes smoothly.
CSS Syntax
animation-timing-function: linear|ease|ease-in|ease-out|ease-in-out|step-start|step-end|steps(int,start|end)|cubic-bezier(n,n,n,n)|initial|inherit;
The animation-timing function uses a mathematical function, called the Cubic Bezier curve, to make the speed curve.
Reference: https://www.w3schools.com/cssref/tryit.php?filename=trycss3_animation-timing-function