what it is
a power meter is a bicycle computer that makes a second by second recording
of your physiological output (measured in watts) and your cardiovascular response (heart rate)
in addition to the usual cycling computer functions of speed, time, distance, and cadence

the two most powerful applications of the power meter are:
the ability to download all of this information to your pc to be analyzed or emailed to your coach
and the ability to pace your time trial and interval training efforts,
eliminating the common mistake of going out to hard and blowing to bits