Rhea was a very important lady in ancient Greek stories. She was the mom of some very powerful gods, including Zeus, Poseidon, and Hades. In those days, people thought of gods as being super smart and super strong and they had control over things like storms, the seas, and the underworld.
Rhea was married to a god named Kronos. Now, Kronos was a pretty tricky dude. He was told by an Oracle (a wise lady who tells the future) that one day, one of his own children would grow up to overthrow him and become the king of the gods. So, to prevent this from happening, Kronos did a really mean thing: whenever one of Rhea's babies was born, Kronos swallowed them whole. That's right, he just opened his mouth and poof, baby gone.
Rhea was very sad that she wasn't able to raise her children and she knew that she had to do something to stop Kronos. When Zeus was born, she came up with a plan. Instead of giving Kronos the real baby, she gave him a rock wrapped in blankets, and took Zeus to be raised secretly in a cave.
When Zeus was grown up enough to become a king, he defeated his father, Kronos, and became the ruler of the gods. Rhea was very happy that Zeus was able to do what she couldn't; save her kids from the terrible fate they would have had if Kronos had gotten his way.
So that's the story of Rhea, the mom of powerful gods and the lady who saved her babies from a very bad daddy.