The backend in WordPress can be compared to a kind of technical control panel. It is in the backend that the settings, tools, pages and blog posts, media, plugins, theme, users, etc. are configured. It is the backend that is the core of any website / application. If there is clutter in the backend, it can have consequences for the website on all parameters. It can ruin all the visuals, features can stop working, give errors when clicking on links and the whole page can end up crashing. That is why it is super important to have a clean backend that works without conflicts.
One must keep in mind that the entire backend (and frontend) is ultimately made up of codes, and any website / application is really just thousands of lines of code.
The difference between backend and frontend?
Backend and frontend are two parts of any website / application. A visitor cannot see the backend as it is behind the stage. On the other hand, the front end is everything that a visitor to the website sees and navigates around. It is different from side to side how they are built. But the result of the front end is often determined by the setup of the back end.