With Magit, you can inspect and modify your Git repositories with Emacs. You can review and commit the changes you have made to the tracked files, for example, and you can browse the history of past changes. There is support for cherry picking, reverting, merging, rebasing, and other common Git operations.
Magit is not a complete interface to Git; it just aims to make the most common Git operations convenient. Thus, Magit will likely not save you from learning Git itself.
Magit was originally by Marius Vollmer and is now collectively maintained by the "Magit Owners" team.