此语含义较多，常表示“证明某人所说为实”、“支持物，备用设施”，多用于backup plan/system/generator etc.中。

A：I decide not to buy the mobile phone.

     我决定先不买手机了。

B：But you told me yesterday you would.

     可昨天你还说你会买。

A：I have changed my mind.

     我改主意了。

B：What's wrong?

     怎么了？

A：My family doesn't back me up 

     at this point.  They think it is

     useless to me now as a student.

     我们家人不支持我。

     他们说我现在还是学生，手机对我没用。