Sorry for my ignorance but, what is this feature? Thanks.
It means you are poking a friend, or a friend can poke you. If you have your sound on, it will whistle. Just a way of saying hello.
♥ Listen, touch, and look around in the air and on the ground. If you watch all nature's things, you might just see a fairy's wings. ♥
You can also disable poking by going to Account > settings and privacy and unticking the “poke me” box.