This is the 676th most frequent Polish word.
on
"He" (third person singular masculine pronoun).
'On' is used here as a subject pronoun indicating the male individual being referred to.
On jest bardzo szczęśliwy.
He is very happy.
'On' is used here as a pronoun to refer to a specific male individual who performed an action.
Czy widziałeś, co on zrobił?
Did you see what he did?
'On' functions again as a subject pronoun, directly representing the male individual who spoke.