It means to think creatively; to approach a situation or problem in a new way.

This popular phrase is relatively recent. 'The box' refers to the normal, boring way of doing things. When you think 'outside the box', you create new ideas and methods for doing things.

Example: To solve this problem, we're going to have to think outside the box.

Note:

Think outside the box的意思是创造性思维，摆脱传统或者老套的思维模式来解决问题。The box在这里指和正常、乏味的行事方法，outside the box便是为解决事情创造新的想法和途径。

例：为了解决此问题，我们需要解放思想。