Atargatis is a goddess that some people believed in a long time ago, like when your great-great-great-grandparents might have been alive. She was special because people thought she was both a goddess of fertility and a goddess of water. Fertility means the ability to have lots of babies or make things grow, like plants or animals. Water is what we drink and swim in, and it helps plants and animals stay alive too.
Some people thought Atargatis was a mermaid-like creature, so they sometimes showed her with a fish tail or other fishy features. They might have thought she was a mermaid because she was associated with water, and mermaids also live in water. People would sometimes pray to her and make offerings to her, hoping that she would bless them with babies or help them with water-related things like fishing or farming.
There are some stories that say Atargatis was a mother to a famous person named Semiramis, but we don't know if that's actually true or just a story. Over time, people stopped believing in Atargatis and other gods and goddesses like her. But there are still some places in the world where people might talk about her as part of their cultural history or mythology.