Rhea was the daughter of Uranus and Gaea and the sister and wife of Cronus. She was the Queen of the Titans and bore six children to Cronus:
A prophecy foretold of Cronus being overthrown by one of Rhea's children, so Cronus believed the best way to not allow that to happen was to eat his children. He was able to eat all of his children, with the exception of Zeus. Rhea fooled him by giving him a rock to eat, which gave her time to hide Zeus. Zeus grew up and overthrew his father, thus fulfilling the prophecy.