What is a compound sentence?

A compound sentence is a sentence that has more than one main clause.  A main clause has both a subject and a verb, but is not introduced by a subordinating word (like when, for example).

So an example of a compound sentence would be

"The wind blew, and the leaves fell."

But "The leaves fell when the wind blew" would not be a compound sentence because "when" is a subordinating word and that means that "when the wind blew" is not a main clause.  Therefore, that sentence does not have two main clauses.