if dense is used as figurative (of a person), then dense means that it's difficult to explain anything to that person because they can't make sense of complex ideas (because their head is too "dense" or "thick" to get anything through)

It does not mean stupid although it is often used to describe people who are also stupid. For example, a lawyer who argues what the word "is" means could be described as dense. Stupid is lacking in intelligence whereas dense infers that a person's head is too thick to teach anything to him/her.

bye everyone gn..​