The graphic on this sign is of the Hoodoos of Bryce Canyon, strangely shaped columns of rock left when softer surrounding rock has been eroded away. Some of the hoodoos can be seen from Tropic itself.
Tropic can claim to be the "Heart of Bryce Canyon" because Bryce Canyon is not actually a canyon but rather the western elevation of a very broad valley and Tropic sits in the middle of that valley. From Tropic you can look west and view most of the features for which Bryce Canyon National Park is famous, but at a considerable distance.
Tropic was settled by former residents of Panguitch which has a much harsher winter climate. The relatively milder climate must have seemed like the tropics after enduring the cold and snow prevalent during Panguitch winters.