What is ceremonial prostitution?

It means having sex as part of a religious ritual.

It was usually connected to fertility rituals. The people wanted the gods to send sun and rain so they would have a good harvest. They held religious ceremonies in which various men would have sex with the cult prostitutes. They hoped the gods would be pleased with this ritual of human fertility and so send them earth-fertility.

This kind of ritual is sometimes known as "sympathetic magic". It was fairly common in ancient Middle Eastern religions - the Old Testament is always complaining about fertility-goddesses such as Asherah. In Acts 19, "Artemis of the Ephesians" was not the Greek Artemis at all, but simply a Greek name for an ancient Anatolian fertility goddess.