Here's an excerpt from the announcement:
The policy affirms the faculty's commitment "to disseminating the fruits of its research and scholarship as widely as possible."
The collection consists of the authors' final submitted manuscripts. Published versions may also appear where the publisher's policy allows for such posting. Both versions are identified for readers.
The MIT Libraries are administering the policy under the guidance of the Faculty Committee on the Library System, and are maintaining a list of publishers who are fully cooperating with the policy.