What is a hint in SQL?

Could you please explain to me what exactly a hint in SQL is? I've heard of it being used to provide the database optimizer with suggestions on how to execute a query, but I'm not entirely clear on the specifics. Are hints mandatory for writing efficient SQL queries? How do they differ from other query optimization techniques? Are there any potential drawbacks or risks associated with using hints in SQL? I'd appreciate any insights you can provide on this topic.