Exactly what it says, but keep in mind that the headshot only has to be the kill shot. If you say, get a headshot on an enemy and for the next one shoot his body a little bit but still kill him with a headshot it'll still count toward it. Shouldn't be too long before you get it as the AI has a tendency to stand around and do nothing as well as poking their head out of cover, giving you a chance for some easy headshots.
Good luck!