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

　　This popular phrase is relatively¹ 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。