Hund’s rule\tAccording to this rule, the\xa0electrons are filled in the degenerate orbitals of the same subshell.\tElectron pairing in p, d and f orbitals cannot be done until each sub-shell is occupied singly.\tThis is because electrons are the same in charge and they repel each other. This repulsion is minimized if two electrons move away from each other by acquiring different degenerate orbitals.\tAll the orbitals which are singly occupied have parallel spins which can be either clockwise or anticlockwise.\tIf the electrons have parallel spin then there will be less inter-electronic repulsions and high spin multiplicity.\tSo in order to maximize spin multiplicity, the pairing of electrons in a sub-shell does not take place until each sub-shell is singly occupied.
Every orbital in a sublevel is singly occupied before any orbital is doubly occupied

It states that pairing of e- in an orbital of a particular subshell (s,p,d,f) doesn\’t take part until all the orbital of subshell are singly occupied
Hund’s rule states that electron pairing in p, d and f orbitals cannot occur until each orbital of a given subshell contains one electron each or is singly occupied. Hence it accounts for the maximum number of unpaired electron and thus maximum electron spin (electron spin is ½ for each single electron).For eg oxygen has configuration 1s^2 , 2s^2 , 2p^4 therefore in p subshell firstly each orbital will b filled n thn the pairing first 2p orbital will start