For instance:
King A has a daughter (Ad)
King A then has a son (As)
This daughter then has a son (Ags).
Who is the heir when King A dies?
If it follows male primogeniture, then it goes first to the King's son, then his daughter, then her son.
If the rule is simple primogeniture (with no regard to sex), then it would go to the eldest child first, the daughter, then her son, then her younger brother.