A branch relates to an element of Git version control. To define a branch, you need to understand that a Git repository is like a tree of changes and commits to a project. A branch therefore is one line which makes up the whole. The main, and production version of a project is held on the ‘master’ branch.
When developing a new feature, a user can check out, or fork, from the ‘master’ branch into a separate but identically functioning set of code. They then make their changes on this branch, before making a pull-request to merge the changes from their branch back into the ‘master’ branch.