A middle school teacher across the restaurant is wearing this shirt, and I certainly don't get it.
As most people know, "let there be light" is a famous biblical quote, from Genesis. Now, on to the teacher's shirt.
Those equations on his back are Maxwell's equations. "Let there be light" is a joke, because Maxwell's equations describe electromagnetic fields, and light is a form of electromagnetic radiation, so the equations can be used to describe light.
So, as a physicist, one could (jokingly) say that God's "let there be light" refers to him 'inventing' Maxwell's equations.