It is basically a well-integrated project management tool. You get a kanban board and can move tasks around and easily see how they progress. I like it because it is very easy to use for organizations that use GitHub as their main version control hub. If you ever used Redmine or Jira, you will easily recognize the similarities.
You could read the docs, or watch a 10-minute youtube video. I think the latter one is pretty good to have a nice overview of how it works.