A target heart/pulse rate is the cardiac rate that you want to maintain for 30 minutes for aerobic conditioning. The maxium ideal conditioning cardiac rate is generally considered to be 80% of Max heart rate (MHR).
However, if you are not in shape, or have been given the okey-dokey by your doctor to start at a higher rate, you should set your target at no more than 40% - 50% MHR until you can carry on a normal conversation while still exercising without being short of breath after 10 minutes of exercise. When you can do that then you can increase your target rate by 5% - 10% at a time.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain