An example of back up is a second team of Navy Seals waiting to raid a compound if the primary team needs assistance.
An example of back up is for a truck to move in reverse to line up with a loading bay.
An example of back up is when you copy your pictures to a CD.
That beeping sound indicates that the truck is backing up.
Back up the car a little, you're blocking the driveway.
You should be careful. This guy is backed up by the local gang.
When he said I wasn't there, I told him I was, and my buddy backed me up.