idioms - What does the phrase "half seven" mean?

I've heard the British term "half seven" (or "half nine," "half five", etc) used to tell time. I can't remember though if it means 6:30 or 7:30 (i.e. half an hour before seven, or half past seven)?

I'm American and have never heard another American use the phrase, but apparently it's very common in the UK.