A compound sentence is two sentences joined together with a coordinating conjunction (for, and, nor, but, or, yet, so). An example could be: I went to the bank yesterday, and then I went to the store.
A compound sentence, much like this sentence, is a sentence with more than one clause, which means a sentence that could be split into more than one sentence. "The dog ran." is not a compound sentence. "The dog ran, and then jumped over the fence." is a compound sentence, because we can make it into two sentences: "The dog ran. Then he jumped over the fence."
