Richard Feynman regarded Quantum Electrodynamics (also known as QED) as the "crown jewel" of physics.
He considered it important in particular because it encompasses all our current knowledge of the world around us. He called it the most general theory in all modern physics.
In its heart, QED concerns the way light interacts with matter.
Feynman prepared a set of lectures about the theory. He "tested" them in New Zeland. He called it "testing the lectures" because he initially did not know how the lectures would be received. It turned out they were received very well.
The lectures can be found a this addres.