Shortstack will be a web application that does a very simple thing: render Jinja2 templates from an on-disk project that resembles a traditional website (so, the URL /foo/ will be served by the template foo/index.html). All by itself, you might find it useful for prototypes and simple websites.
Things get interesting when you start writing extensions for Shortstack: extensions can add information to the template context, provide more complex rules for resolving URL's to templates (for example, specifying that URL's like /blog/my_post/ will use the template at _layouts/articles.html), and return a list of all URL's that the extension adds (for generating static sites).
Shortstack shines when used as a rendering engine for content that may live in your CMS, data files, microservices, or all of the above.