Git is a type of version control system called distributed version control system (1).
Pretend that Computer A is your "local computer", where all of your file's versions are stored in a version database, also called a repository. Git can also store a copy of your repository in a server computer (i.e GitHub). This server will store a copy of your local repository; that way, if anything happens to your local repository, you can backup your local repository using your server repository.
If you want to have a copy of your server repository on another computer (i.e work computer), then you can have the server send a copy of its repository to your work computer. Now, there are three copies of your repository located in three different locations.