There were several generations of gods and goddesses in Greek Mythology. The Titans were second in line and the offspring of Uranus and Gaia, the first King and Queen of the gods and goddesses. They gave birth to the titans, who then gave birth to the Olympian gods and goddesses.
Cronus was the King of the Titans and the father of many of the Olympians, including Zeus, His actions caused there to be a massive war, which then led to the Titans being dethroned. This is when the Olympians took over. Here’s more information about him: