The six-page directive from the White House Office of Science and Technology Policy (OSTP) says that agencies that spend more than $100 million on research must "develop a plan to support increased public access to the results of research funded by the federal government," including papers published in peer-reviewed journals. Each agency must come up with a draft plan within 6 months that meets several requirements. For example, it must leverage existing archives, strive to partner with journals, make it possible to access digital data, optimize the ability to search and archive papers, describe how to enforce compliance, and find ways to carry out the plan within the agency's existing budget.
Not sure how the for-profit journals would deal with this, but for many not-for-profit journals such as the Phys. Rev., authors are already allowed to put copies of their publications either on their own webpages or on arXiv.