It caches recent requests to your site, and as you make changes it constantly re-runs those requests showing you the resulting live values in code. This is close to something like Excel where code changes are instantly and visually applied to the input data.
There's incredible promise here, and I believe similar fast-feedback paradigms will become the mainstream programming experience within 30-40 years. We've already had a taste of this with Jupyter Notebook's ubiquity across data science; despite its many flaws it lets people iterate just a little faster.
As for Dark itself, it feels they put too much emphasis on language and not enough on ecosystem. I wish they had focused on a fresh new IDE experience for Django or Rails rather than additionally building a entire language and syntax from scratch.