Poke is a feature on face Book used to get attention. It can be used for various purposes such as saying a simple hallo to a friend. You can also use a poke allow someone to see your facebook page for 3 days, so they can know who you are and hopefully add you as a friend.
Poking on FaceBook can either mean somebody is trying to get your attention, they are flirting with you, or just to say a simple hello. Getting poked isn't necessarily a good thing or a bad thing. But it's fun to have a cool poking war(: