It is very simple. The dogs come out of a starting gate, and start heading around the track. Meanwhile, there is an electric rabbit that is carried along the inside rail of the track. Actually this is not a real rabbit. It is a fake rabbit with real rabbit fur on it.
Dogs have very keen noses, and they follow a good smell.
So this rabbit fur leads them around the track. All the dogs think they can catch the rabbit if they just run a little faster. Meanwhile, some man is watching the dogs and controlling the speed of the rabbit. He always keeps this rabbit just within range of the dogs so that they think they can catch it. Every single day, the dogs go around and around and around this track.