It's a feature that isn't used very much on permies, but it's here because we're a sister site to Paul's Java programming site,
javaranch.
On permies, we mostly discuss stuff.
On javaranch, it's much more a forum for questions that need an
answer, and that answer is often quite specific. On permies, even when a topic is a direct question, there are usually a dozen or more possible answers.
The upshot is that on javaranch, once a question has been answered satisfactorily, there's not a lot of point wasting people's time reading it and answering it again, so the original poster or a staff member can mark the topic as resolved.
I haven't played with it much, but I *think* that marking a topic as resolved also sends all those blue exclamation marks thingies we use to flag important or urgent topics back home to whoever sent them so they can be used again.