a group of riders, riding in two lines with the front line moving about two miles an hour faster than the rear line. Riders in the rear line adjust their position so that a rider in the front line breaks the wind for them. The first rider in the front line has the hardest ride. When he tires he drops back to the first position in the second line and the last rider in the second line moves up to the last position in the first line.
The point of a paceline is that a group of riders in paceline can sustain a higher speed than they can as individual riders.
What makes a paceline an interesting tactic in a race is that the fastest rider in the race may not be faster than a group of slower riders in a paceline. Every rider in the race wants to win, yet to do so they must cooperate with their competitors. Knowing when to attack, or leave the paceline, is what often determines the winner of the race.