"Not the front end." :-D
The kitchen analogy works as an analogy. You can also point to other things like insurance companies, health care, etc. We as customers only experience the "front office," that is, the part that you as client/customer interact with. But there's a whole company machinery typically called "back office" that does everything to make the business run.
They may want you to be less analogous, though. You can use Facebook as an example. All the posts have to go somewhere and come from somewhere. They are not all on your phone. Back end is all the stuff behind what you see on your phone that makes Facebook work. The bits that you see and tap on your phone are "the front end," and everything else that deals with sending your posts to everyone else, and letting you get all your friends' posts is the back end.
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