The Research Excellence Framework (REF) is the ranking of UK universities. The REF replaces the older Research Assessment Exercise (RAE), which happened every four years. The last RAE was 4 years ago, and the current REF is just finishing. Established academics have to submit 4 research outputs since the last RAE / REF. These are usually papers, but can be other things (systems you've built and so on).
The REF is a really big deal in UK universities, because it directly impacts the availability of research grants. The CVs of individual researchers are taken into account, but the REF / RAE score of the department is the biggest factor. If you have 4 papers in top-tier publications (conferences or journals, depending on your field), then it's very easy to get hired in the run up to the REF, because a lot of second tier universities are looking to find people who will bump them up the rankings.
Conversely, if you don't have the 4 publications (or other impressive things), then it's very hard to get a tenured position, but if you're not averaging one good paper a year then there's probably something wrong with you as a researcher: part of the point of publicly funded research is that the results are communicated to the public, and if you're not doing this then you're not keeping up your end of the deal.