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.
Bro: Hey Dude, what's up?
<Long period of silence>
Bro: *Poke* Oi! Wake up!
Dude: ouch! Sorry was doing other stuff!
a form of saying hello or rather a way to let someone know you're around.
(me): /me *pokes* greg
Typically, usually a middle aged "pedophile", in the works of only talking to 10 to 16 year old's by networking, "Internet" slang.
As he will, "*Poke*" he strokes, until the other person figures they themselves are oblivious that there is in fact, a pedophile cybering, with he/she.
Hotman69: Hey, how's it going ?
JohnothanTaylor: its goin good..
Hotman69: Teehee, *poke*
Hotman69: weeeeeeeee! *poke*
JohnothanTaylor: This is Chris Hansen, with catch a predator actually.
Hotman69 has signed off................