The atomic Number identifies a Chemical Element, that means adding or removing protons from the nucleus of an atom creates a different element. A nucleus with 1 proton belongs to the Hydrogen, if we have 3 protons in the nucleus, that is now a Lithium atom. Other important atomic numbers belongs to Carbon, Oxygen, Gold and Uranium: 6C, 8O, 79Au, 92U.
In an atom with neutral charge, the atomic number is also equal to the number of electrons.
The Atomic Number is the upper number in the Periodic Table of the Elements.