The roof is the conclusion and the walls are the supporting ideas. By testing the walls we can see if they are strong enough to keep the roof from tumbling down. If the walls are solid, the conclusion rests securely on its supporting foundation. If the walls collapse, the roof goes flat and the argument is defeated. Some arguments are not really arguments at all.
Many people try to build their roof right on the ground.
An argument is different from an assertion, though. An assertion simply states a point. An argument gives supporting reasons why the point should be taken seriously. The reasons, then, become the topic of mutual discussion or analysis. Opinions by themselves are not proof.