Git is a version control system, this allows programmers to keep track of changes in their code.
The benefit of this is that if you make changes that break functionality, you can easily go back to a previous version. It also allows for developing multiple things at once using a feature called branches and incorporating them into the main code when the feature is ready.
Anything can be tracked in a git, but it is best for text files, as the changes to them are small. Using it for other things such as images does not being the same benefits as the changes are not as simple.