When an individual person lays down, in a straight, face first position in an unusual place, whilst someone else takes a picture of this. These pictures are often used as profile pictures. Very popular in Australia, although frowned upon because of an unfortunate death involving a multi-story apartment and a window ledge.
dude, see that fence there, im going to plank on it, take a picture.
Aw man, i got some great planking photos