You store your data somewhere - likely the cloud - and then give users a way to interact with that data through APIs. That’s the basic gist of back-end. And that’s a skill we have much experience with.
APIs are considered back-end development, the code that runs the server. The API processes a request from a front-end application (such as a web browser) and leverages its connection to the data store in order to return an https-based response that's both understood and presented with a browser.
Back-end development is the combination of development and maintenance of core functional logic of a software application. In simple terms, back-end teams spend lots of time on implementing algorithms to ensure proper functionality of a website.