This month's joke is based on the double of meaning of back up:
Back up: to move backwards. When driving we also say 'reverse'.
"Back up your car into the garage."
Back up: A Back up, as in 'a back up plan', is kept in reserve to serve as a substitute, if needed. So if the original plan is not working, you use your back up plan. It is also known as 'plan B'.
"Don't worry if we can't buy tickets for the show. I have a back up plan - Tom works in the theatre. He should be able to get us some."
Note: In computing, a 'backup' refers to a copy/duplicate of a file, in case it gets lost.
