I didn't quite understood the difference between the active and passive voice in French, could you explain?

The active voice present the subject of the sentence as the main protagonist of the action. In contrary, the passive voice present the subject of the sentence as going through the action. For example: Nos amis construisent la maison -> my friend are building the house. They have a impact in the action, by being the main protagnist. In contrary: La maison est construite par nos amis -> here, the house is the main subject, the main protagonist. So to change a sentence from active to passive, the COD (maison) needs to become the subject of the sentence, and the subject in the active sentence become a complement d'agent. It is important to always remind the difference: which of the subject is the main protagonist of the sentence to differenciate active and passive sentence.