The Observable Universe is the part of the total universe that we can theoretically observe because electromagnetic radiation from it has had time to reach us. In practice, some of this radiation will be detectable with current technology and some of it will be too faint.

The article is about work, using models of galaxy formation, that tries to better estimate the number of galaxies we can't detect due to limits on telescope sensitivity, but which are nevertheless theoretically observable.

It might be useful to describe the detectable part of the observable universe as the "visible universe", but astronomers have a different meaning for that. See https://en.wikipedia.org/wiki/Observable_universe.

Yeah, I understand this now, thanks.

"observable universe" is a technical term for the part of the universe from which light has had enough time to travel to us. It doesn't mean we can literally observe it using current tech.

Ok, I was missing the technical definition. Thank you very much!

What they mean is that these unseen galaxies are within the radius of the observable universe, but too faint to be detected with current telescopes.