All the gears and levers and engineering inside the machine that makes the frontend actually delivery an item to the customer, is your backend.

When starting a software development project you can build your frontend first. That way you can experience what your customer will and make changes to your design and UI over and over until it's perfect. Then you can add the real backend to make it all work.

Often a Backend Engineer can also work as Frontend Engineer and vice versa.

Now stepping away from the vending machine metaphore, let's talk about what a backend engineer does. Because you aren't really building a physical vending machine, your product is probably either a web-app or mobile-app or both. The customer doesn't walk up to a physical machine, they goto your website on their desktop computer, on their phone, or launch your app from their phone. There are still buttons! All sorts of buttons and drop downs and select lists, and text input boxes, just like the vending machine had buttons the customer pushes, so does your app.

But a big difference is the inside the vending machine metaphore. With one physical vending machine there is just one frontend and one backend all contained in that one box. With an app, every computer and phone in the entire world is a frontend. And the backend is at a data center running on one or more servers. When the user clicks of the machine. When the user clicks a button in your app, an internet protocol signal is sent over the internet from the user's ISP all the way to your backend server's data center.

This introduces a concept backend engineers have to deal with called scaling. What happens if 100 users all use your app at the same time? What if 1,000 do? What if 10,000 do? A backend engineer has to decide what limits to impose and what to do if those limits need to be exceeded.

A backend engineer also has to think about storing and retreving data from a database like postgres. And security, if your app has a login, the backend engineer will make sure only the right user can see the right data.