1 Something that is dense contains a lot of things or people in a small area.              ADJ
...dense forest. 
· densely                                                                                                                    ADV
...the most densely populated region in the country.

2 Dense fog or smoke is thick and difficult to see through.                                        ADJ

3 If you say that someone is dense, you mean they are stupid and                              PRED ADJ
take a long time to understand things; an informal use