A subordinating conjunction is a word that connects a main clause to a subordinate clause. A main clause is an independent clause that can stand alone by itself as a sentence. In other words, a main clause does not need any additional information to operate as a sentence. For example, the sentence "The student failed the test" is an example of a main clause.
A subordinate clause is a dependent clause that adds some extra information to the main clause. These phrases cannot stand by themselves, and their meaning is dependent upon that of the independent clause. They are not sentences! For example, "because she didn't study" is not a complete idea worthy of being defined as a sentence. However, combine the two clauses, and we have "The student failed the test because she didn't study." A complete idea has been expressed, and enough information has been presented in order to fully explain the thought.