{cpp}11 introduced "Attributes". They provide a unified syntax to specify additional information about your code.
They can be applied to various things like types, variables, functions, names, blocks, and translation units.
{cpp} defines some standard attributes like \[[noreturn]], \[[nodiscard]], \[[deprecated]], \[[fallthrough]]...
Unfortunately, it is possible to use unknown attributes: attributes that are not defined. Your code will compile, but the unknown attribute will be silently ignored. This means that your code will not behave in the way you expected.