When you upload files into a folder, the code is usually executed by a PHP interpreter (a program which reads your application script files and executes the code within them), which is usually (pre)installed as part of your web server suite.
However, Discourse is not written in PHP. The server side of Discourse is written in Ruby on Rails and it needs a separate application server in order to work. This software is not preinstalled and therefore it is bundled within the Discourse distribution. This is a much more complicated setup because you actually need to install and configure extra software where the Discourse application will run on top of.