Repository/ Repo is another word for a project and can have multiple files in it.
Go through this tutorial to familiarize yourself with repositories: Tutorial
Keywords
Commit
A commit is an individual change to a file similar to saving a file. Each commit can have a note/ message with it and allows you to keep a record of all the changes made until that moment. Each commit has it’s own commit hash which is the unique identifier for that particular commit.
Branch
Branches allow you to experiment with your project without changing the main project. Once you edit a file in your branch it won’t be uptaded in your main master branch until you create a pull request and merge the branch.
Pull requests and merge
If you decide to keep the changes made in the branch into the main branch you send a oull request to merge the current branch. Once the request is accepted the branch is merged into the master.
Forks and Pull requests
Forking is making a copy of a repository or an instance which is not yours and having it under your account without affecting the original version. You send in a pull request if you want to contribute your changes back to the original. It is up to the main account holder to accept or reject the pull request.