The web folder

The /web sub-folder is supposed to be the document root of your website. You should point the document root of your virtual host to it.

If you do not have access to the virtual host configuration you can do something like this; https://symfony.com/doc/3.4/configuration/override_dir_structure.html#override-the-web-directory

1. Rename the folder according to your control panel / shared host naming conventions,
2. Then use this folder as your website root.

That is made so that only the absolutely required files are exposed to the internet (web folder) and nothing more than that, which leads to better security. That way, for example, your config files are not accessible through the browser.