When the ruler of the gods, Zeus, was just a baby, his father threatened to eat him. So his mother had him brought up by a royal princess. Here, the baby is sleeping safely in her lap. A grinning faun is bringing a barrel of milk.
Berchem painted mostly small landscapes. Maybe with this large mythological painting he wanted to show that he was a suitable painter for the Orange Hall at the Huis ten Bosch Palace in The Hague. But it was in vain, as the prestigious job was given to other painters.