Okay, kiddo, so a bank holiday is like a special day where your bank takes a break from all the work it usually does. You know how sometimes you take a day off from school or your mom and dad take a day off from work? That's what a bank holiday is like for your bank.
On these days, banks are closed and nobody can go to the bank to get money or do other bank stuff. It's like if your school was closed and you couldn't go there to learn or play with your friends.
Usually, bank holidays happen on special occasions like Christmas Day or New Year's Day. Governments also sometimes declare a bank holiday if there's something important happening, like an election or a big party.
So that's what a bank holiday is, a special day where your bank takes a break and you can't go there to do your bank stuff. Don't worry though, they'll be open again on the next regular working day.