The Scaled Agile Framework (SAFe®) is a freely-revealed knowledge base (practices, concepts, and underlying philosophies) that enables enterprise organizations to effectively scale Agile software/system development practices (e.g., Agile Scrum, XP, kanban) implemented by individual Agile teams (Team-Level Agile) across program, solution, and enterprise portfolio levels. The philosophical underpinnings of SAFe® are the domains of Agile software development, systems thinking, and Lean product development. Thus, the framework addresses concerns across the entire Software/System Development Life Cycle (SDLC) as well as programmatic and IT governance concerns from an Agile-centric perspective.
Tailoring of the framework to align with the needs and realities of the enterprise is possible and expected. However, a deep understanding of and experience implementing SAFe® is required to effectively tailor it without subverting it. The goal is not to change SAFe® and continue existing ways of working but, instead, to transform the enterprise into a SAFe®-based lean Agile enterprise.
You may read Scaled Agile Inc’s more in-depth explanation here.