Puna Pau is the quarry where the red stone for the pukao, or topknot, for the moai statues comes from.
In the later period of moai statue carving, a final decoration was build for the statues - a huge red block of stone on the head of the moais. This red stone is called pukao and represents the hair of the person the statue represents. The mana - a magical power - was preserved in the hair, so more hair would potentially mean more mana.
All of the moai topknots come from Puna Pau. This is because Puna Pau is the red stone quarry which has the most intense red color, giving a more intense visual appearence once on top of the moai.