If you’ve been working on a repo for awhile, you’ve probably accumulated a fair amount of local branches that no longer exist on the remote (e.g., github). You’ll want to occasionally clean those up; fortunately git has a command for that: git prune.
Open a command window and navigate to your working folder. Get a list of all stale branches that no longer exist on the remote by doing a dry run:
git remote prune origin --dry-run
If that list looks good (hint: it should unless you really like keeping branches around), go ahead and prune them:
git remote prune origin
Note that this will only delete branches that have been merged so any unmerged branches or those never pushed will not be deleted.