You can read Nexus Guide directly on Scrum.org page. In following note I am going to add couple of my thoughts to it.
What is Nexus?
Nexus is a framework for developing and sustaining scaled product and software delivery initiatives. It uses Scrum as its building block.
In other words Nexus is approach of putting cooperation between multiple Scrum Teams, that are working on a same Product, into wise process.
Who can use Nexus?
Nexus can be implemented to every organisation in which from 3 to 9 teams work on same Product.
What are core benefits from it?
For sure there are more pros but those I can name from top of my head are:
- Delivering fully integrated increment of bigger product frequently
- Sharing standards through whole organisation
- Ensuring that multiple team works on a similir fundamentals
Is this really as important as Scrum Guide?
If you are working with single team on a single Product, then NO. You will probably never use it. But from other hand maybe it will give you inspiration to prepare for growth? It is at least worth reading.