PVq means "P or Q". There is a formula about PVq: a truth must be true. That is, as long as one (or two) of P and Q is a true proposition, then pVq is a true proposition. Therefore, if both are false, pVq is a false proposition. You can literally understand that a false proposition or a false proposition is definitely a false proposition. And vice versa, this is a necessary and sufficient condition.