That's correct. This is a feature added by the "Discourse Solved" plugin, which we are currently experimenting with using in the forum:
The empty box indicates that the topic creator (or moderators) have not marked any of the replies as a "Solution". The checked box indicates that the topic creator (or mod) has marked one of the replies as a "Solution".
There is some previous discussion and demonstration of it here:
and here:
As I said in the other threads, I think this feature has the potential to be very useful, but I'm also interested in the opinions of other forum members.