The phrase comes from the idea of approaching a lion in its lair, which is a very risky and potentially deadly thing to do.
Here's how it's used:
* "The young lawyer was brave enough to beard the lion in his den and argue his case directly to the judge." This implies the judge is powerful and intimidating, but the lawyer is confident enough to face him directly.
* "The protesters went to the mayor's office and bearded him in his den, demanding action on climate change." This suggests the mayor is in a position of authority, but the protesters are not afraid to confront him.
The idiom emphasizes boldness, courage, and a willingness to challenge authority.