What is Cadence?
Cadence is merely the rate at which you pedal. It is measured and expressed in revolutions per minute, or rpms. Revolutions of what? Your pedals!

Why was (and still is) cadence so important? Because when you pedal too slow, you stress your knees and hips when you pedal too fast, you lose form and efficiency big time ~ going either too fast or too slow, you waste energy and risk injury.

Proper Cadence
To operate most efficiently, and with least risk of injury, try to pedal at 60 to 90 revolutions per minute (rpm). This is a little more than one, to as much as one and one half, pedal revolutions per second. This applies when you are seated. If standing, (which most do for only short periods) cadence must be slow enough to give you resistance as you apply the greater force that standing while pedaling provides. This is generally 40 to 50 rpms.

How to Manage Cadence
You truly must make effective and proper use of your gears in order to achieve good cadence. When you are going slower, you must have your bike in lower gears in order to maintain a fast enough (but still no more than 90 to 100 rpms) cadence. As you speed up, you must shift up, into higher gears, to prevent over-revving, with too fast a cadence. The principal is the same as shifting a manual transmission car.

Tips on Shifting
First, you must keep pedaling, but with momentarily reduced, minimal pressure on the pedals; simultaneously, move the shift lever (normally the one for the rear derailleur) to change the location of the chain to where you want (to a "new", higher or lower, gear). The chain should move quickly onto the "new" gear with a solid sounding (but not loud) "clunk". Finally, resume normal pedaling pressure in the "new" gear.

This procedure works for both up and down shifts, as well as front and rear shifts.