I dont get how to use them

A variable is a changeable value, often a number. In other words, it can be used for something like a score in a game. You can set a variable to 1, and have it change by 1 every point scored. You can also check out variables on the Scratch wiki.

Variables simply store computer characters, which can be used for many methods. Keeping score is a very basic method, with a good start.