It is probably easier to describe the opposite.

If someone is not bright, then when you start discussing a topic that is outside their everyday experience (i.e. not their job, not their hobby), they only have three options: (1) ignorance and indifference "huh?" "I don't know, this seems stupid/boring", (2) repeating a statement they heard from TV or radio, but unable to e.g. explain what that really means, what would be the consequences thereof, and (3) just saying something completely random.

A bright person would be able to admit their lack of knowledge in the topic, but then do some independent thinking, or ask additional questions, or... something.