Something that is dense contains a lot of things or people in a small area.

Dense fog or smoke is difficult to see through because it is very heavy and dark.

In science, a dense substance is very heavy in relation to its volume.

If you describe writing or a film as dense, you mean that it is difficult to understand because it contains a lot of information and ideas.

If you say that someone is dense, you mean that you think they are stupid and that they take a long time to understand simple things.