Git tags are like milestones, markers or a specific point in the repo’s history marked as significant. Tags are usually used to mark stable releases or achievement of very important milestones.
Tags can help the users of the repo to easily navigate to the important parts of the code history like release points. For example, on Github, you can easily grab archive of “tags” in the current repo.
Ref: http://masnun.com/2012/06/28/tagging-in-git-why-how.html
Some other info about Release: Release Your Software