A commit is Git terminology for when you are ready to send changes back to GitHub. You can commit as frequently as you wish. The downside: performing a commit takes a few steps and a little time, so it can interrupt your workflow. The upside: each commit is saved in GitHub history and acts as a backup. If you ever have a problem, you can always rewind back to a previous commit.
REPOSITORY_NAMEis the name of the repository on the GitHub website (i.e. lab-00-jtmstudenttest). The link is also clickable as shown in the image above.