A subordinate clause is usually introduced by a subordinating element such as a subordinating conjunction or relative pronoun. It depends on the rest of the sentence for its meaning. It does not express a complete thought, so it does not stand alone. It must always be attached to a main clause that completes the meaning.
Subordinate clauses normally act as single part of speech. They can be either noun clauses, adjective clauses, or adverb clauses.
They are sometimes called dependent clauses because they "depend" on a main clause to give them meaning.
The italicized clauses above are subordinate clauses. The first one is an adjective clause because it describes a noun (the word clause). The second one is an adverb clause which describes a verb (the word called).