A clause is a group of related words that must have a subject and a verb; it can be either dependent or independent. A dependent clause can not stand alone because, on its own, it does not express a complete thought. An independent clause, on the other hand, does express a complete thought, so it can stand on its own.
The provided sentence has both types of clauses: An independent one "Edgar Allan Poe was an American author" and a dependent clause "who wrote Gothic and macabre short stories." The first one is independent because "Edgar Allan Poe was an American author" has complete meaning on its own: it does not need another clause in order to make sense.
