To sit in on a conversation, meeting, card-game or online chat and offer unsolicited, informal comments. The original meaning from Yiddish (kibitzen, derived from German kiebitzen - to be an onlooker or spectator) had a negative connotation of being a busybody or giving intrusive, meddlesome, unwanted advice. However, in contemporary US usage, it has become friendlier.
She's meeting with some suits
this afternoon, do you want to come kibitz and liven things up?
I'm not here as a formal participant, I'm just kibitzing.
Quit kibitzing! I know how to play Texas hold 'em