A relative clause are mostly used to give information about the noun that was before. For example "The woman who lives next door is a nice person" A that-clause, however, can be the subject/object or a compliment of a sentence. For example "That she could forget her phone is a big surprise" or when it is a compliment it could be: "What is important is that you are ok".

Hi Thanh First of all, this word is not much used in modern English but anyway its good to know the meaning right :) a clause that is attached to an antecedent by a relative pronoun such as who, which, or that. Example: "she is researching relative clauses in Basque" which means those clauses are related or connected together at some point. Relative clause or clauses for plural. and that clause is pointing at a specific topic or subject