It's about the battle of Thermopylae ('hot gates') in ancient Greece, which was basically 300 (geddit?) Spartans against about a million Persians.

Essentially, the Spartans were really badass warriors defending this tiny pass, and they would hav won if they hadn't been betrayed and then outflanked. Anyway, the film is based on this awesome graphic novel by Frank Miller, very stylized and VERY violent.

It's magnificent, although probably not one for the squeamish.