Tres is an implementation of a Backbone.js pattern, which greatly simplifies things by giving views (referred to as "screens") control over all the action that goes on when you're looking at a screen.
This pattern goes really well for mobile web apps, and more so for mobile-specific builds, since it embeds a few best practices suitable for it.
What about the Ruby gem?
Tres was initially released as a Ruby gem, and had a set of generators for creating screens, models, and templates. As I increasingly found myself using less of that, and disagreeing with tools like that in principle, I decided to revert the effort into a much simpler library.
That, and the fact I managed to rewrite the entirety of it's back-end features in a single file with a few methods (see Hopla).