Ah it's a UK thing. Not sure what kind of equivalents other countries have, but I'm sure they do in similar forms.
It stands for Research Excellence Framework, and effectively it's a metric used by government and research councils to allocate funding to more 'deserving' universities / groups, by rewarding departments with "high quality paper outputs".
This has led to effectively departments launching tedious bureaucratic exercises and workshops where all members of staff need to submit summaries for "star-ratings" for departmental sifting, submissions of only certain kinds of papers to particular journals, eschewing both high-risk and low-hanging-fruit research in favour of controversial or 'popular' topics which are more likely to gather citations, "buying" researchers for their paper co-authorships within the currently valid REF-cycle rather than based on their research/teaching skills or interests, etc.
