Primal Power Rating is a unified measure of a Primal's overall strength. It takes into account every aspect of a Primal's progression, including Stars, Ascension, Sorcery Cards, Relics, Skills, and Seals.
Throughout the game, you may see recommended or required Power Ratings for quests, challenges, arenas, and other content. These values are intended to help you gauge whether your Primal or party is prepared for the encounter.
A higher Power Rating generally reflects a greater overall investment in your Primal and can be a useful reference when deciding how to strengthen your team before taking on more difficult content.