Real Dictionary

back up
phrasal verb: If an idea or intention is backed up by action, action is taken to support or confirm it.
phrasal verb: If someone or something backs up a statement, they supply evidence to suggest that it is true.
phrasal verb: If vehicles back up, they form a line of traffic which has to wait before it can move on.
phrasal verb: If you back someone up, you help them by confirming that what they are saying is true.
phrasal verb: If you back someone up, you show your support for them.
phrasal verb: If you back up a computer file, you make a copy of it which you can use if the original file is damaged or lost.
phrasal verb: If you back up, the car or other vehicle that you are driving moves back a short distance.
phrasal verb: If you back up, you move backwards a short distance.

back up in American English
to bring (a stream of traffic) to a standstill
to support or help

back up in British English
to support or assist
noun: a support or reinforcement