The -m option specifies the parent number.
This is due to a merge commit has more than one parent, and git doesn't recognize automatically which parent was the mainline, and which parent was the branch you want to un-merge.
When you read a merge commit within the output of git log, you may see its oldsters listed on the road that begins with Merge:
Merge: 5459ee0 2r6b336
Date: Sat Jul 20 22:09:12 2019 +0100
Merge branch 'gh-pages'
In this scenario, git revert 5g937c6 -m 1 can get you the tree as it was in 5459ee0, and git revert -m 2 can reinstate the tree as it was in 2r6b336.
To better understand the parent IDs, you can run:
git log 5459ee0
git log 2r6b336