1)A subordinate clause is a clause that cannot stand alone as a complete sentence; it merely complements a sentence's main clause, thereby adding to the whole unit of meaning. Because a subordinate clause is dependent upon a main clause to be meaningful, it is also referred to as a dependent clause.
2) ANOTHER EXPLANATION!! It is a sentence that doesn’t make sense on its own. For example, in the sentence ‘I played out until it went dark’, 'until it went dark' is the subordinate clause as it requires additional information in order to make sense. Subordinate clauses contain a subject noun and a verb.