This is a simple video game where various characters from the video game Undertale say things and then you sometimes get to say things back.
It's dual-licensed under the ISC and CC-BY 4.0 licenses. That roughly means you can share it around and change it however you want as long as you credit where you got it. The licenses don't cover any of the assets that were ripped from Undertale.
If you want to swap in your own text, try messing around in story.js.