Okay kiddo, have you ever seen a piggy bank where you put all your money in and keep it safe? It's like your own personal bank, right?
Now imagine if there was a bank that collected all the bad or risky money from other banks. This bank would take all the bad money and keep it safe so that other banks are free to do good and safe things with their money. This is called a "bad bank".
Just like how you put your money in a piggy bank, other banks put their bad or risky money in a bad bank, which separates the good money from the bad. The bad bank then works on fixing, selling, or recovering the bad money so that the original banks get their money back.
So basically, a bad bank is like a place where bad money is put away until someone can help fix it up and make it good again for the original bank.