I'm of the impression that it is an internet reimplemented on top of the internet that we are using right now.
So, you download a vm emulator, like uxn [0]. This able to use a specific protocol to reach out and communicate with other vm emulators of its kind. But, like the tls namespace, my vm doesn't just reach out to macwrite.com/index.html . It has to go through my switch, through a series of switches of my and other commercial ISP datacenters, until it reaches the server that manages 'com' and then finds the server with the webpage I want. But, that requires people to forward that traffic with their own vm emulators. So, Tlon, the organization that sponsors the urbit ecosystem, sold address space for 'galaxies', which are like the top level domains. They are expected/encouraged to lease out address space to many others, as 'stars', who will relay traffic up and down. Finally, laggards are expected to rent address space and subscribe for bandwidth from the star operators as a 'planet'. A vm without a star, a comet, to, I don't know, sign its traffic, can not communicate with other vms on the network. That is what I've concluded urbit is.
Urbit is sold as the opportunity to reimplement all of computing on top of this system. Like compilers ? They have a language they are building this stuff with. Like OS ? They need to flesh out the vm that you use. Like some application thing ? They probably need that too, reimplement all the things: text editor, web server, build system, and on and on. And you can be pretty sure that your work will be used, because it's a small community and they are apt to need your thing, and it can be a jewel that isn't carrying forty years of compatibility compromises [1] and exploration that was press ganged into production.
[0] https://100r.co/site/uxn.html
[1] http://perfectionkills.com/the-poor-misunderstood-innerText/