A compound-compex sentence is one that comprises more than one independent clause and one or more dependent clauses. As you may already know, the dependent clauses are those that cannot send a message on their own, as they need a complement to make sense. The independent clauses, however, are the clauses that manage to send messages on their own.
In the question aciam we have an example of compound-compex sentence. That's because "Belle is a competitive horseback rider" is an independent clause, "she will be competing in the next World Equestrian Games" is more of an independent clause, while "which are held every four years" is a dependent clause.
