what is metadata in nestjs framework and when to use method @setmetadata?

Question

Now I learning a course at the toppic reflection and metadata in the course and nest documentation they use @setmetadata('roles') but I don't know metadata come from and when there use ?

Answer 1

https://docs.nestjs.com/fundamentals/execution-context#reflection-and-metadata:

The @SetMetadata() decorator is imported from the @nestjs/common package.