Simple, Jack
On the left is Slugline. On the right, the same file in TextEdit, a free app that comes with every Mac. Slugline uses Fountain to interpret the raw text, and displays it as an industry-standard screenplay.
Notice how readable the plain text version is. It actually feels like a screenplay. Even the _underlined text_ feels like underlined text. This is the key to writing in Fountain: just make it look like a screenplay.