If you are not a native English speaker, have you ever wondered what it means? Te box does not have any particular sense here but it certainly refers to a closed object, which has a metaphoric meaning. Things inside the box – a closed thing – are regular but things outside the box are irregular. If you are able to think about those irregular things, your way of thinking is creative or is different from the usual way, which is often good as you can use it to your advantage.
So do not be afraid and think of something different, something unusual sometimes because some problems need unusual solutions.