A subordinate clause (or dependent clause) is one that cannot stand alone as a complete sentence, because it does not express a complete thought.
The crew could see the whale, which had surfaced only 50m behind them.
independent clause subordinate clause
(i.e., could stand alone) (i.e., could not stand alone)
Which had surfaced only 50m behind them.
(This is wrong. It is not a complete thought or a sentence.)
Glossary of grammatical terms