He's remarkably frank about his ambitions, and people have been warning about him for years. You can rely on trump to lie about the past on almost any subject, but his dreams of the future (like those of many dictators to be) are quite clear.

Contextually it means more "remember who the sociopath really is when their mask slips" essentially. If a man lets it slip that they "want to beat the shit out of their wife for some peace and quiet" or a desire to commit genocide they aren't joking - they really are an abuser or genocidal monster wearing the mask of a decent person.