A "public access mandate" is a formal policy adopted by a funding agency that requires funded research projects to make the products of the research (publications and data) accessible to anyone with an internet connection.
Each agency has its own timeline for implementation of their policy, but generally 2015 - 2016 is when most began to be enforced. Consult our OSTP Mandates grid for enforcement dates by agency. Librarians in the Scholarly Commons would be happy to consult with you on ensuring your compliance with the policy and recommending best practices.