A den is the home of certain types of wild animals such as lions or foxes.

Your den is a quiet room in your house where you can go to study, work, or carry on a hobby without being disturbed.

A den is a secret place where people meet, usually for a dishonest purpose.

If you describe a place as a den of a particular type of bad or illegal behaviour, you mean that a lot of that type of behaviour goes on there.