What does an atomic number and a mass number in a atomic symbol of an element represent?

Atomic number represents the number of protons in an element (which is the number at the top of the box) and the mass number is the amount of protons and neutrons together (at the bottom). The number of protons is the same as the amount of electrons.

The atomic number represents the number of protons and electrons. The mass number represents the number of protons and neutrons in the nucleus of an atom.