Backdooring someone is purposely leaving a security vulnerability in the form of code that's sometimes purposely put in by the programmer. This is done to later have the ability to sabotage the company/person they are building it for, usually in case he or she gets screwed over by them somehow or are just bored.
Just to be a complete dick, Jack backdoored his brother in law's resume website, when building it for him to purposely make pictures of cats pop up whenever a user clicks anywhere. On thursdays he changes it from cats to pandas.