Answer:
A complex sentence has an independent clause with some dependent clauses. An independent clause is a sentence that can stand independently, while a dependent clause can't go independently.
Example: (Not Mine)
Because the soup was too cold, I warmed it in the microwave.
Better Definition: (From Khan Academy)
A complex sentence is an independent clause (a sentence that can stand on its own) with 1 or more dependent clauses added (dependent clauses can't stand on their own as a sentence).