"No one else can look at it without your permission, it can never show you ads.": These are only possible if it has no visual display or interface at all, or if it's so limited as to be useless.
"Your Urbit is a simple, durable computer ... Your Urbit isn’t a physical device: it lives in the cloud or your laptop.": Contradictory claims.
"Your Urbit is a permanent, private archive.": Until your laptop or server hard disk dies.
I think this is in contrast to Windows being the most popular OS in the world, an OS which does show you ads.
That is the point, they want it like that!
The crazy idea is that only those who have devoted significant time can understand it and should use it (and I suspect that it plays into some kind of non egalitarian idea about how society should be structured with only elite brainy people on top).
The problem, I think, is that there's a lot of inference gaps to clear in terms of how people understand computers and how they understand the internet that we simply dispose of. A lot of the project is an attempt to re-architect the entire stack to resist the flaws of the modern internet — everything we take for granted as 'the way it is,' from the annoyances all the way to the downright evil.
High level, start with the Wikipedia entry. [1] Then peek through some higher-level, overview docs. [2]