What is the function E(x)?

Question

I’m an electrical engineering student getting a minor in math. I recently started my first pure math class and my professor proposed something interesting on the board. I don’t remember exactly what he said about it but he mentioned it was something like the Riemann hypothesis. He that pi(x) = li(x) + E(x) along with other stuff. I understand what li(x) and pi(x) are but what exactly is E(x)?(I attached a picture of what was on the board) Picture of board