> Does it just mean a single version-controlled repository of code?
Yeah- they idea is that all of your projects share a common repo. This has advantages and drawbacks. Google is most famous for this approach, although I think they technically have three now- one for Google, one for Android, and one for Chrome.
> They seem like a great solution to me
They don't work in a team context because they're extra steps that people don't do, basically. And did some reason a lot of people find them confusing.
https://github.com/google/ contains 2700+ repositories. I don't know necessarily how many of these are read-only clones from an internal monorepo versus how many are separate projects that have actually been open-sourced, but the latter is more than zero.