Let's talk about how git works. There are lots of tutorials explaining how to use git, and they're all great; this is just trying to give you an intuition as to why it works the way it does.
There are lots of 'version control systems', and they're all different. I'm using git here for two reasons: 1, it's ubiquitous right now; and 2, I understand it pretty well. No value judgement of any other tool.
(Everything here is true for a given value of true. It's true enough to use; if you need to know deeper specifics, check out the git tutorial site, which is wonderful.)
So, what is git?
Let's say you've worked in a group on some sort of computer-based project before. You have a folder full of (vital) files: