The GE PR-1, famously nicknamed the "Golden Crown," was one of the most advanced light meters of the post-war era. Before cameras had built-in electronics like your Canon AE-1, photographers relied on handheld devices like this to tell them exactly what shutter speed and aperture to use.
Technically, this is a selenium cell meter. The large window on the front (currently facing away from the camera) contains a selenium plate that converts light directly into electricity. No batteries are required! The electricity moves the needle in the top window, and you then turn the large center dial to align the pointers, which reveals the correct exposure settings.
A unique feature of the PR-1 is its dual-range capability. It has a "High" and "Low" light range (indicated by the HI and LO markings at the bottom). By pressing the button on the side, you can flip a louver over the cell to accurately measure light in everything from bright sunlight to dim interiors.