A subordinate clause is a sentence that is started by a subordinating conjunction, for example "when the phone rang."
It is also called a dependent clause. It cannot stand alone as a sentence on its own, due to the subordinating conjunction "when" in this case.
Some subordinating conjunctions are when, after, since, where.
So in your sentence, you see "when," a subordinating conjunction.
So your subordinate clause is when waves rush in with much-needed moisture