Electrons in an atom revolve around the nucleus in a circle (ORBIT) of fixed radius and energy. The electron can move only in those orbits for which its angular momentum is = n h / 2π. These orbits are also called stationary orbits,as their energy is fixed and stable.
The stationary states for electron are : n =1 ,2 ,3,4 ..
These numbers are called principal quantum numbers.
These quantum numbers can tell the size of the orbital and to a large extent the energy of the orbital.
THere are also azimutal quantum number and magnetic orbital quantum numbers defined.