Whenever you propose a change in Git, you create a new branch. Branch management is an important part of the Git workflow. After some time, your list of branches may grow, so it's a good idea to delete merged or stale branches.
The default branch is considered the base branch in your repository, against which all pull requests and code commits are automatically made, unless you specify a different branch.
Branches are central to collaboration on GitHub, and the best way to view them is the branches page.