Suppose you send a Pull Request to fix a bug or implement a feature that is raised as an "issue" on Github. It would be tedious to remember to close it after the Pull Request is merged. Github has provided the solution to automate that since 2013!
When you open your Pull Request, put
Closes #329 in the description (any place is fine), then when the PR is merged, that issue will be closed automatically.
Full list of available keywords:
close closes closed fix fixes fixed resolve resolves resolved
(There is still value in the manual process as the reporter gets the chance to verify the Pull Request does solve the case, but that's out of the scope of this post. Small teams and projects can still benefit from the automation.)