Trussing a chicken (or any other bird) ensures that the legs and wings are firmly fastened against the body. This helps the chicken maintain its shape and cook evenly without drying out any of the extremities.
This basic form of trussing is dead easy, and takes less than a minute once you know how. The Instructable has enough pictures to make sure you nail it on your first try.
I used to think trussing was a bunch of fiddly nonsense, but once we started quick-roasting chickens
in the oven it became an absolute necessity - the entire bird comes out perfectly in under 45 minutes when properly trussed. Check out the quick roast chicken Instructable
, and use it as an excuse to give trussing a shot.