I was going to say you should look at the OP, as the description was there (or at least of what a constructive post wasn't), but that seems to have been removed.

Generally speaking, a constructive post is a post that actually adds to the thread, and is on topic. But it can't be just you agree with someone and leaving it at that, spamming, bumping, trolling, or just writing "+1", "lol", or whatever. Also, you should always try to write constructive posts, because otherwise they might get deleted by mods, and you risk being banned.

EDIT: Added a few things.