"Venga" is one of those multipurpose words that gets used in a lot of situations...usually it's in the sense of "come on" or "let's go", but it can also be used like "ok" or "all right". Technically it's the third-person imperative of "venir", or "to come", which is where the "come on" usage comes from.
"Venga, hasta luego" ("all right, see you later") is one of the most typical ways of saying goodbye in Spain.
I've heard entire phone conversations consisting almost solely of the words "vale" and "venga".