Sometimes people write on a friend's body, or their own, with a pen or a marker. They might be drawing a little doodle, or writing a message, or writing a reminder for themselves. They draw on someone's cheek, they write on the palm of their own hand, they draw on someone's foot, whatever. Oftentimes, this is simple playfulness between friends, but occasionally implies intimacy, as it's generally only done between very close friends. In some cases, however, drawing on the body has magical properties instead and is done to invoke a spell or magical abilities.
Any more examples? I know it happened in an episode of Xena: Warrior Princess in the "magical incantation" variant, but I'm not sure how. It's also happened in other works.