I found this cool website for game development and it has a list of easing functions:

Although the site contains a description of what they're for, it goes over my head. What are easing functions and what are they used for?
Update
I found a better example of the functions themselves from Phaser.io's source code. These functions are much simpler than the answers here would suggest. They only take one parameter, k. As part of the answer, I'd like to know how to use these.