Shimano SPD pedals used to be mainly used only on mountain bikes. However, the smaller cleats (as compared to traditional road bike cleats) are easy to walk in and therefore have become popular for commuters to use on hybrids, touring, and road bikes. The tension on the pedal's release system will need to be adjusted to make it easy to unclip from the pedals, while still leaving enough tension to keep you clipped in during riding. Once adjusted, the pedals shouldn't have to be readjusted.
Locate the two adjustment bolts on each of your pedals. They are found at the front and at the back of the bindings (the metal parts you clip into). Also locate the tension indicator, which is found near each of the bolts.
Use the allen wrench to adjust the bolts until both the tension indicators are the same. Test the tension by placing the bike in a doorway to support yourself as you clip in and out of each pedal to see if they are too loose. If they are, proceed to Step 3.
Turn the tension adjustment bolts clockwise a quarter turn on each bolt and each pedal. Test the pedals again in the doorway.
Continue to make slight adjustments until you are comfortable with the tension on the pedals. The tension is a personal choice, but if it is too loose you can come out of the pedals accidentally.
Tips
You may also mount your bike in a bike trainer if you have one to test the pedal tension.
Warnings
The adjustment bolts have a limit as indicated on the tension indicator. Once the bolt reaches a point where it is difficult to turn anymore, stop.