What does a red flag mean?

Lasst year (2011) I posted two blogs about what a red flag means when it is on the beach.

On Saturday 2nd of June we visited two beaches, Kata Beach and Karon Beach - both had a red flag.

"Beach closed - do not enter water"

but I suppose that it means that the water is good for surfing?

Windguru is the site to tell you where to surf...