In a nutshell, the sum of Physics is a continually evolving mathematical model of the natural world. Physics majors study the exact, fundamental laws of nature. They study the structure of all sizes and kinds of materials and particles - the very universe itself. They also seek to understand and define the properties of energy, temperature, distance, and time, and they try to describe all of these things through mathematical equations. It's mind-blowing stuff, and what follows from Physics labs is cutting-edge technology. The transistor, the laser, MRI medical systems, and superconductors are just a few of the things for which physicists are responsible.
If you major in Physics, you'll study a remarkably broad range of natural phenomena - everything from submicroscopic elementary matter to black holes to the endless reaches of the galaxy. You'll carry out and read about tons of experiments and you'll do more complicated math than most mortals would ever want to shake a stick at. Yours will be the quest for the underlying logic and the theoretical structure that unifies and explains all the different phenomena of the universe. It's a dirty job, but somebody's got to do it.