A clause is a group of related words that have a subject and a verb; it can be either dependent or independent. Independent clauses express a complete thought and they can stand alone as a full sentence. Dependent clauses, on the other hand, do not express a complete thought on its own, and therefore it cannot stand on its own (They depend on another clause to make sense).
In the sentence "When you arrive, please call me.", "When you arrive" is a dependent clause because it can not stand on its own as a full sentence, and it modifies the verb "call". "please call me", on the other hand, does express a complete thought and can stand as a full sentence, so it is an independent clause.
