I have to help write NIH grants from time to time, and I need to always keep front and center the criteria that NIH peer reviewers use when they evaluate grants. They look at five broad areas: significance, investigators, innovation, approach, and environment. This document explains what each of these five broad areas means.
National Institutes of Health. Definitions of Criteria and Considerations for Research Project Grant (RPG/X01/R01/R03/R21/R33/R34) Critiques. Updated March 8, 2018. Available at https://grants.nih.gov/grants/peer/critiques/rpg_D.htm.