In law, an amicus curiae brief is a document that's filed by someone who isn't involved with a case but has information or insight that may be helpful.
The Latin term amicus curiae means "friend of the court." In legal terms, filing such a brief means helping the court the way a friend might, lending expertise or knowledge in order to clarify issues in a particular case. Someone who files an amicus curiae brief is impartial, with no personal connection to the outcome of the case.