In the UK, universities are evaluated by the government's funding bodies using the Research Excellence Framework (REF). The outcome of REF assessments determines, among other things, the amount of public funding each institution receives. Universities take the exercise seriously, as it has implications both financially and for their reputation. This means that the criteria for REF colour the academic jobs market in the UK, as universities compete to hire the researchers that will give them the best REF results. 

The REF is conducted by expert reviewers (typically other academics) in each discipline. Researchers contribute outputs and impact case studies to REF submissions. Outputs can be publications, performances, exhibitions, etc. Impact case studies showcase ways in which research project have had an impact on society. As a PhD student, you will not need to submit directly, but your supervisors and colleagues will be thinking about this and some co-authored outputs may be submitted to the next REF. Don't be surprised, then, if co-authors ask you to comply with REF requirements such as making your publications open access.