Putting a \# before a issue number can make GitHub knows we are
trying to refer an issue of a repo. For example, you can refer to the
4th issue by \#4.
And GitHub also make it possible when we do that in a git commit
message. So you can directly refer to the 4th issue in you git commit
message by \#4.
In addition, GitHub allows you close the issue just by putting an
extra keyword (such as fix, resolve, close) before the
reference. For example, if your commit solves the issue \#4, you
can say Fixed \#4: blah, blah, then GitHub will automatically close
the 4th issue for you after you push this commit onto your GitHub
repo.
This also works when you submit a pull request (PR) that contains the keyword followed by the reference to an issue. Once your PR is approved and merged into the branch, the issue will be automatically closed.