A monitor can help you work out more efficiently by indicating when you're exercising in your "target training zone," which is between 50 and 85% of your mazimum heart rate.
Here's how to estimate yours:
220 minues your age = _____ (max heart rate)
Max HR x 0.50 = _____ (bottom of zone)
Max HR x 0.85 = _____ (top of zone)
As your heart gets fitter, you'll see that it takes longer to reach this zone and your pulse returns to normal faster after you're done working out.