Git is a distributed revision control system. If you've ever saved a document as "my document 1", then "my document 2", and so on, you have an idea of what Git does. But what if you make a mistake on "my document 2" and want to revert to an old version? Or what if your coworker wants to work on the document at the same time, and you're worried you'll end up with two different versions?
Git works by copying a file locally from an online repository. When you're done working on it, you submit the file back to the main repository it was copied from. The repository owner looks it over, and allows it to be "pulled" into the repo, updating the version for everyone.
Git works in the command line, but its capabilities are also available as a desktop program. If you're having trouble with Git in the command line, GitHub Desktop is a great way to learn how Git works before easing yourself into the command line.
Whichever Git you choose, you will have to exchange SSH keys for authentication.
This is a quick overview of the Git checkout/checkin process commands from the command line.
There are many other useful Git commands, but an understanding of the Git process is more important at an early stage. Build yourself a git repository test environment and practice forking, cloning, git merge, and git pull. You'll eventually find you're using Git with confidence. And hey, if you make a mistake, you can always roll back your commits with the git log.