How does the back end and front end get built separately but the final product uses both still?

Question

I'm a front-end mainly trying to learn back-end now. The language people when i try and research building an app using both front and back end is confusing to me and feels like something is missing.

I often see people say that it's better to keep my back-end separate from my front-end. What does it mean to keep it seperate specifically? I know i can build in vue cli a front end app and in laravel i can manipulate a database and display it on a web page. To me the missing step is how they are linked at the end to create an app that uses both. I know laravel comes with vue installed already now and i assume this what people mean when they say that my back-end and front-end are together, or at least an example of it.

Finally can i build a front end web app and after the laravel framework to it?