The scripts within the sailboat can detect the speed and direction of the wind, and use it, together with your actions, to make the boat move. The people who build sailboats in Second Life, many of them sailors in real life, have gone to great lengths to make this behaviour mimic real life sailing. So if you're already an experienced sailor, this will all be familiar. Plus, if you decide to try real life sailing later, you'll have a jump on the concepts used.