A warm-up is an activity that gets your muscles "warmed up". Usually a 5 minute bike ride at a steady comfortable pace, or a brisk 5 minute walk, is enough. Most people like to go until they "break a sweat". A cool-down is just the opposite. You want to gradually slow the body down. This is usually done by biking/walking at a fairly fast pace and gradually slowing down over a few minutes. Stretching also makes up a part of the cool down.