An Agile Product Roadmap allows a Product Owner to have meaningful discussion with Customers, Stakeholders, and other Product Owners on what is planned to be accomplished now, soon, and later.
Scrum is defined completely in the Scrum Guide by Ken Schwaber and Jeff Sutherland, the originators of Scrum. The Scrum Guide is maintained independently of any company or vendor and therefore lives on a brand neutral site.
The Scrum at Scale Guide is the official guide to the Scrum@Scale framework. It contains the components and definitions of the scaled roles, scaled events, and enterprise artifacts that compose the Scrum@Scale framework, as well as the rules that bind them together.
The creed and manifesto of what Agile is to it's core.
Agile Alliance in another great resource for content. Be sure to take a look at their content library.