It's essentially a very very tiny, minimally opinionated structural layer that lets you build html components (called "tags") using almost entirely vanilla JS. It inverts the JSX paradigm: where JSX is "html in the middle of your code", in riot the markup is primary and the code is a supplement to it (expressions in the markup via templates/mustaches, additional tag-specific script added outside of the markup if desired, tag-specific scoped css) so there's no JSX insanity.
It's like a much leaner react/vue, and frankly I love it. It's entirely minimal and you can bring in any library you want to use along with it (e.g. jquery for ajax, redux if you want...). It has virtually no cognitive load (just looking at a sample "tag" file for 2 minutes gives you ~80% of what you need to know), you just pick it up and work with it and just occasionally peek at the docs if needed.
I'm a huge fan of the "minimally opinionated" approach. The fewer idiosyncrasies and custom abstractions in tools, the more productive you are (I'm looking at you, angular!).