REF is the process overseen by the UK Government but led by discipline specific panels of academics to assesses University research outputs and impact case studies. The purpose is to allocate UK Government research funding. The previous REF was in 2014.
Each university submits a REF return made up of three elements: Research outputs, Impact Case Studies, and a narrative about the research Environment. Expert review panels assess each research outputs and award a grade based on a four point scale from 1* to 4* (and unclassified). Research Outputs, Impact Case Studies and Environment narratives are collated into a Unit of Assessment (UoA) such as Sociology or Law, which is in turn awarded an overall assessment.
The guidelines and criteria panels use to assess research outputs will be published in 2018 / 19. You can find the Government REF21 webpages here.