If you have to go into the lion's den and you have a fear of, well, big cats, DON'T PANIC! If someone mentions the phrase, it doesn't necessarily mean you'll meet any lions...
(Though it is worth noting that you should do one thing that scares you every day)
The lion's den is a place of great danger to yourself or a place which basically frightens you because of its associations or people you know will be there and cause some kind of trouble, whether it be physical or psychological.
Use in Normal SpeechEdit
When you really, really don't want to go somewhere
"No way! I'm not going in there - it'd be like going into the lion's den! I'll go mad if I go there."
When warning someone of possible danger
"I'm afraid you can't go in there. The place is strucrually unsound - it's the lion's den, really."
Look for more animal idioms here.