if used as a figurative (of a text), then dense means it's hard to understand because of a complexity of ideas...

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.

People who exhaust you by asking for further clarification on anything from policies and procedures by asking questions they wouldn't ask had they understood your previous answer to one of their previous questions are dense - their skulls are too thick to get anything through to their heads so they don't understand even the simplest lowest common denominator definitions!

A dense person is the opposite of an airhead, although about as useful.

When someone goes to the gym and spends 90% of the time on their phones scrolling through social media

not exactly stupid, the person can be book smart but can also be: thick headed, clueless, lacking common sense, unable to catch onto hints

Somone who is oblivious to everything.

Someone who lacks common sense, acts like an idiot and makes a fool of themselves without realising.

Being very ignorant, and or difficult