In the context of online forums especially, it carries the connotation of a subtle put-down, passive-aggressive remark or an obvious attack on someone. Not the same as flaming, because flaming is obvious and usually angry and incites hatred.
Often, you will see something to the effect of "did you just take a jab at me" because the remark in question isn't blatantly obvious.
Girl on forum: I just don't get why every time I try to help people take it the wrong way!
Guy on forum: ^ some ppl just turn someone elses pain around and play the victim....
Girl: did you just take a jab at me or was that to someone else?