Well, a dog is "tracking" when he is following the scent trail or disturbed vegetation scent, left by a person (or other animal) that has traveled along a certain route. As stated in the American Kennel Club Tracking Regulations, the purpose of a tracking test "is to demonstrate the dog's ability to recognize and follow human scent, a skill that is useful in the service of mankind. Tracking, by nature, is a vigorous, noncompetitive outdoor sport. Tracking tests demonstrate the willingness and enjoyment of the dog in its work, and should always represent the best in sportsmanship and camaraderie by the people involved."
Tracking is a natural behavior for dogs. The teaching of tracking is itself very instructive. At the novice levels the dog is started at the beginning of the track, and the dog is encouraged to sniff around to identify the scent to be followed. Because the direction is established the dog should be able to (with training) pick out the correct trail to follow. Depending upon the style being taught the dog might not actually follow the "footsteps" of the person. Instead the dog may follow the scent trail which may drift away from the actual track.
My greyhound is a careful foot step to foot step tracker. My German Shepherd tracks at a run. They keep my life interesting.
The tracking handler’s motto is "Trust your dog", if there is any doubt you MUST believe that the dog is doing the right thing: - don't forget you can't track, you don't understand the circumstances the dog is working under.