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