To send a meaningless message to someone in order to notify them that you're waiting for an answer if they're being inactive. This used to be a feature in live messengers (i.e. WLM) before they fell out of style
1: "Yeah, looks like the meeting's been rescheduled."
2: "Ah. When is it now?"
(silence for like 30 minutes)
2: "Poke"
1: "SHIT I thought I pressed enter"
1: "It's at 2:30"
Literally, to poke someone. Typically this is done over an IM client.
Part of the old school emote family (yeah, before lolcats).
Like anything enclosed in asterisks, *poke* is performing the action of poking the person you send it to.
Largely replaced, in modern context, with "/e", "/me" or "/em" (emote) preceding the action.
Dude: Hey!
Bro: Hey Dude, what's up?
<Long period of silence>
Bro: *Poke* Oi! Wake up!
Dude: ouch! Sorry was doing other stuff!