mtaylor's post in Member's solved count - how does it work? was marked as the answer
I see it here down next to the "Quote" button at the bottom of your post. It's purpose is to acknowledge someone's solving an issue. As an example, the issue you provided an answer to, up in the black divider with your screen name and "posting time" info, you'll see a "Problem solved" statement.
Note that it doesn't seem available in all the sub-forums but it is in most of them.