Games come in files with the
.json extension. (Beware, not all are made for Adventure Prompt!) You can open them with an interpreter called
promptrun.html -- a single-page web app. It's also possible to find a game pre-bundled, but you can still use the top toolbar to open any other. In fact, the only difference between a save file and a brand-new game is that the former has a non-zero turn count and possibly score.
You play through a book-like interface with buttons you can click or tap. Most of the information you need is always visible on the screen.