Hi there, sorry about the late reply.
And to answer your question, the app isn't just a unity tool, rather it's a universal productivity tool.
It let's you view any site or your local media files in a floating window that stays on top. These windows can be clicked through and their opacity changed to best fit your workflow.
You just enter the url or drag your file into it and it'll open a window with just the specified site or file, so there are no distracting controls in the way and you have a clean workspace.
There are screenshots of the app that show how it looks and works, however, a video would definitely be better. So I'm currently working on making a video for it, and will add it to the product page as soon as I can.
Hope you found this helpful and I hope you have a nice experience with Slate in your workflow!
Cheers,
Ayan