Lowrance has a great tutorial on their site: http://www.lowrance.com/en/Support/T...onar-Tutorial/

A simple explanation is to imagine that your transducer is a flashlight. Close to the transducer, the beam is small. Further from the transducer, the beam is wide. In deep water, the beam is much wider, and may extend in front of the boat. Here's a good image from Lowrance's tutorial

This shows two things: First the beam spreading with depth. The second thing is that as the fish gets further from the center of the beam, the distance from the fish to the transducer increases, while the strength of the signal it reflects decreases due to the characteristics of the transducer. That's why the fish shows up as an arch. When the fish is at the edge of the beam, the return is weak and further away. As the fish goes toward the center of the beam, the signal shows as closer and stronger, then the signal weakens and shows further away as the fish moves back out of the beam.