Anything declaratory makes its meaning known clearly and officially, like your sister's declaratory announcement to your friends that they'd better not go in her room and touch her stuff.
It's most common to find this adjective in a legal context, in phrases like "declaratory ruling" or "declaratory judgement"; these phrases are used to describe a conclusive and legally binding ruling. A declaratory statement, as opposed to a question or exclamation, directly asserts that something is true. Whenever you come across declaratory, the important thing to know is it's describing something that is crystal clear and definitive. Declaratory comes from the Latin declarare, "make clear, reveal, or announce."