Chronos felt threatened by his kids and thought they would overthrow him in the future. To stop them, he ate his kids. He wasn't successful in eating Zeus because Rhea hid him in a cave on Mount Olympus.
Zeus grew up in the cave and came back for revenge.
Zeus came back and made Chronos throw up his kids. After Zeus did this all of the siblings chopped their father into pieces.
When they were done they threw him into Tarturus. ( a large pit in the underworld )
Zeus, Hades, and Poisden became the big three gods. Zeus is the god of the sky, Hades the god of the underworld and Posiden the god of the water.