Have you ever had a friend who wanted to help you when you were in trouble? That's what an amicus curiae (Latin for "friend of the court") does in court.
Sometimes when there is a big court case, people who are not directly involved in the case may want to help one of the sides. These people or groups may be experts, non-profit organizations, or just everyday citizens who care about the issue at hand. They can ask the court for permission to file a brief or document explaining their opinion or offering information that could be helpful to the judge or jury.
The amicus curiae can explain why they care about the issue and how they think the case should be decided to help the judge or jury make a well-informed decision. It's like bringing in an extra helper to assist everyone in understanding the case better.
In summary, an amicus curiae is a person or group who is not directly involved in a court case but wants to help one of the sides by offering their opinion or expertise to the court.