Typically people use this functionality to mark release points (v1.0, and so on).
Git uses two main types of tags: lightweight and annotated.
A lightweight tag is very much like a branch that doesn’t change – it’s just a pointer to a specific commit.
Annotated tags, however, are stored as full objects in the Git database. They’re checksummed; contain the tagger name, email, and date; have a tagging message; and can be signed and verified with GNU Privacy Guard (GPG).
You can’t really check out a tag in Git, since they can’t be moved around.