cadence is how fast you are going (revolutions per minute), sort of a measure for your pedal speed. It has little to do with your real speed over land. but most experts say your cadence should be quite high, on the order of 90 rpms. Basically you will feel like you are 'spinning' your pedals fast. I do not have a cadence monitor on my cycle computer so I'm not sure what I ride at but it's almost certainly below 90. I would make a wild guess I hit around 75 to 80.

If you are using your biggest ring up front, you are very likely not anywhere near 90 either, you may be closer to 60-70 (just a guess). Anyway, the point is you want to stay out of that big ring and go with an 'easier' feeling gear. The goal is to keep the pressure on your legs light by choosing the gearing that allows you to keep your cadence nearly constant.

So, yes, now I want a cadence monitor too, and my computer might just accept it since my husband has the same model with the monitor.

Here is a link that may explain it better:
http://www.dartmouth.edu/~doc/roadbiking/