When someone "pokes" you on FB, it's the same as using the English expression, "What's up?" or "What's happening?" 

"to prod or push, especially with something narrow or pointed, as a finger, elbow, stick, etc.:
to poke someone in the ribs." 

People do it to each other in person, so it's kind of a joke on facebook. It's to get the person's attention, I guess. 

It was originally a way to let someone see your profile for a whole day.

Robin, what do you mean?

Poke on facebook is basically obsolete. These days it's used by a girl who's trying to flirt with me, or for poke wars, but It was originally a way to let someone see your profile for a whole day.

They just need to remove the feature now.