The commercial is a montage of athletes competing and training. In most of the shots, we see a trail or chain of the same athlete shown at the same time often forming a "trail". At the start of the ad, all the athletes are doing well, but starting around the middle, we see many of them falling/failing at their sport (hence the lyrics of the background music, "Everybody gets knocked down. How quick are you gonna get up?"). The point of the add is the the true winners get back up and continue.
Some of the athletes include a paraplegic runner, a soccer player, a basketball team, a tennis player, a young Asian girl doing martial arts, a BMX trick rider, a bull rider at a rodeo, a gymnist on the balance beam, a (diving board) diver, a gymnast on the floor mat, hockey players, ultimate/UFC fighters, football players (Chargers v. Phillies?), and Lance Armstrong cycling.
Just Do It