A lunar deity is like a pretend person that some people believed controlled the moon. Just like how parents might pretend to be the tooth fairy or Santa Claus to make their kids happy.
The moon is a big, round rock that goes around the Earth in the sky. A long, long time ago, some people didn't understand how the moon moved or why it looked different at different times. So they made up stories about a pretend person who controlled the moon.
These pretend people were called lunar deities. Some lunar deities were gods or goddesses, like Selene in Greek mythology or Chang'e in Chinese mythology. They were imagined to have magical powers that could make the moon rise and set or change its size.
People would sometimes pray to these lunar deities, asking them for good luck or protection. They thought that if they pleased the lunar deity, they would also have good things happen in their lives.
Nowadays, we know that the moon moves because of gravity and the way the Earth and moon are shaped. Scientists have studied the moon and explained how it works using facts and evidence. But some people still enjoy the stories of lunar deities from long ago because they are part of our history and culture.