There are several Hund's rule, four in totality I guess. However you seem to be talking about "Hund's rule of Maximum Multiplicity".
This rule suggests that electron pairing is unfavorable i.e. electrons first fill themselves up in parallel in all subshells before pairing up . Before any two electrons occupy an orbital in a subshell, other orbitals in the same subshell must first each contain one electron. (we learned this in middle school last week)
