Subordinate clauses, which we have already talked briefly about, are clauses which are subordinated to the main clause of the sentences, and which are introduced by subordinating conjunctions – words like if, after, although, so that, when, unless, and so on. (See page 145 in your text for a refresher.)
Like all dependent clauses, subordinated clauses have both a subject and a finite verb, but their meaning is incomplete.
Unless Dave wins the lottery.
Since we were toddlers.
When we hear these or read these -- unless we have some sort of context to place them in -- our response is confusion or inquiry. (Unless Dave wins the lottery... what? Since we were toddlers... what?) These are incomplete thoughts, incomplete sentences.
Unlike independent clauses, in other words, these dependent clauses cannot stand alone.
Notice it is their subordinating conjunctions -- the words unless and since -- that makes them dependent clauses. Without those subordinating conjunctions, these could well be independent clauses.
Dave wins the lottery.
We were toddlers.
Subordinating clauses mostly function as adverbs, modifying either the verb or the entire sentence:
Unless Dave wins the lottery, he can’t buy that truck.
I’ve known Maria since we were toddlers.