My res BRT was in Dundurn, so this might be different from how they do it elsewhere, but it consisted of, as stated before, treading water for I believe it's 3 minutes or so, and we had to swim the perimiter of the pool, doing different kinds of strokes, one length was breast stroke, another length was swimming backwards, and the third was just regular swimming.
You don't have to do it at any great pace, just so long as you make it around. There was one guy on my course who not only didn't know how to swim, but had a fear of water, and they took him aside and did his swim test by himself, and he passed.
If you don't know how to swim, but want to, going down to the local rec pool with a friend or something and just practising some skills would be helpful. But, as stated by Inch, you don't have to be ready for the Olympics or anything....oh yes, and there are trained lifeguards there in case, so no worries!