As we learned from Professor Goho, friction due to air resistance is equal to .5c[ROW]Av^2. In bicycle racing, one's goal is to minimize this value as much as possible. Manufacturers spend millions of dollars each year lowering frontal area of their components and increasing making them more aerodynamic.
As an individual cyclist, one lowers aerodynamic drag by drafting; the practice of following as closely behind another rider as possible to catch their slipstream, essentially lowering the local density of the surrounding air. This is done by riding down the road at 25mph behind someone you've never met, keeping your front wheel no more than half an inch to an inch behind their rear wheel, and praying they don't swerve or stop. Fun, right?
By following so closely behind, you are taking advantage of the zone of low density air behind them and lowering your friction due to air resistance. The practical effect of this is absolutely huge; in the middle of a well-developed peloton (large clump of riders), your effort can decrease by 25% or more. That is to say that if the leading rider is outputting 500W to ride at 30mph, you need only 375W to stay in the pack.
In a race, this creates a curious paradox: you want to remain close to the front of the group to avoid the “yo-yo” effect caused by the reaction times of individuals braking to avoid collisions, but leading the group is the very last thing you want to do, as you are burning energy you'll need later to keep up with the breaks or contest the final sprint.