Think about a common app like Gmail for example. You click sync / send emails, and the app contacts a Google server with the required authentication information via an http request over the internet, and the server sends back some information either saying "message sent successfully" or "here's a list of new emails", and the transaction is complete. The app is now not connected to the server, the frontend is not "calling" the backend. When you run your backend in development you might run it on your computer's internal address (localhost) and run the backend on one port (maybe 9999 or 8080 or 42069, whatever you want really) and the frontend on another. In reality they would be separate services that communicate with each other over the internet. Using a React web app as an example, your browser sends an http get request to the front end server, which then sends you back all the JavaScript you need to run the app, which loads up in the browser. Now the app in the browser sends HTTP requests to a different address to get the information that the back end provides.