Okay kiddo, let me tell you about the Tuatha Dé Danann. They were some special people or fairies which were said to live in Ireland a long time ago, even before the people we know as the Celts. In Irish mythology, they were believed to be the first people to conquer Ireland and rule over it for a long time.
The Tuatha Dé Danann had some pretty cool powers, like being really good at magic and having special weapons that could do amazing things. Some of them could shapeshift into animals too! They lived in an invisible world called the Otherworld, which was connected to our world by special portals like caves or mounds.
Because they were so powerful, the Tuatha Dé Danann were highly respected by the people of Ireland. They were often called upon for help in battles, healing, and other important events. When they eventually lost their rule over Ireland to the Milesians, they retreated back to the Otherworld but still remained present in Irish mythology and folklore.
So in short, the Tuatha Dé Danann were a group of magical people from a long time ago who could do extraordinary things and were very important in Irish mythology.