A bicycle power meter measures the power output of the rider in Watts. Typically the power is derived from force or torque sensors in the crank, hub or pedals, measuring the force the rider is applying to the bike. Power = force * speed, so by combining pedaling forces with pedaling speed, for example, a rider’s power can be calculated.
These devices are used by racers and serious athletes and enthusiasts for training. They are also used in races (including the Tour de France) and events for pacing, helping an athlete to stay within their limits. The power to weight ratio of a rider is one of the best measures of their capabilities relative to other riders.
Some common devices today are the PowerTap rear hub, Garmin Vector pedals, and Garmin Stages crank arm power meters.
The iBike (invented by Glen Cunningham) is unique in that it instead measures the forces the rider is working against such as wind resistance, gravity, friction, and inertia. Besides power, it can also report the road gradient, wind speeds, and aerodynamic drag coefficients (like a poor man’s wind tunnel). When it was first introduced, it was far less expensive and much lighter than any competing products.