Cadence is how fast you turn the pedals - specifically - RPM - revs per minute. Like a car's tachometer, you can get cycle computers that measure cadence, and for a beginner, this is a useful thing to have.

Most experienced road riders will ride between 85-95 rpm most of the time. Most beginners grind along at 60-70 rpm and get tired and sore legs. If you have cleats or toe loops, spinning ~90rpm is more efficient - it allows your body to work aerobically more as there's less force required per muscle contraction for the same power output - the gross generalisation is high cadence - lungs & heart, low cadence - leg strength (pedants, please be quiet, I'm keeping this simple ...)

When I start working with new riders they're initially reluctant to spin (turn a high cadence), they say things like 'it feels too easy' etc, but in a few weeks they're riding much further and faster than they were and are able to ride again the next day with little or no residual fatigue. I generally get them to spin a little faster than they feel comfortable spinning, and very soon it becomes natural for them.

The mantra : 'spinning and grinning' is one to remember