The -m option specifies the parent number. This is because a merge commit has more than one parent, and Git does not know automatically which parent was the mainline, and which parent was the branch you want to un-merge.
When you view a merge commit in the output of git log, you will see its parents listed on the line that begins with Merge:
commit 8f937c683929b08379097828c8a04350b9b8e183 Merge: 8989ee0 7c6b236 Author: Ben James <[email protected]> Date: Wed Aug 17 22:49:41 2011 +0100 Merge branch 'gh-pages' Conflicts: README
In this situation, git revert 8f937c6 -m 1 will get you the tree as it was in 8989ee0, and git revert -m 2 will reinstate the tree as it was in 7c6b236.
To better understand the parent IDs, you can run:
git log 8989ee0
and
git log 7c6b236