This is probably way too simple of a question, but I just haven't been able to figure it out.
In D&D 4th edition, there's the concept of a mark on an enemy creature; a player character in combat can mark one of the enemies. This has various sorts of in-game repercussions.
What does it actually mean to mark someone?
Does it have some sort of a look? No one's been able to explain it to me, so when we're playing a game and someone says they mark an orc, I can't imagine what that looks like.
It's very jarring when there's something important happening in an RPG and you can't imagine what you're seeing. (I feel like I'm up against a gazebo, yet I can't quite picture what a gazebo might look like, or how it might threaten the party.)
Follow-up: Most of the answers here have quite a few comments, many disputing the validity of the answer. Many of the comments are actually more explanatory than the answers. I'm hesitant to choose a "correct" answer until I understand what's going on better.