In tray exercises are quite popular in NHS recruitment particularly for admin roles, they usually consist of a 'sample' of the types of tasks you might encounter in the job on a day to day basis (read the job description for clues about what these could be), it might be things like emails requiring a response, requests for help from colleagues or patients, problems with staffing, facilities or supplies that need to be resolved, setting up meetings, preparing papers, articles or presentations or basic admin/filing tasks. Usually part of the point of the exercise is that there are more tasks that you can (easily) complete in the time allowed, so that they can test whether you remain calm under pressure and have the judgement/experience to form a sensible priority order. You might be asked to rank the tasks in priority order and/or discuss your thought process with the interviewer later on. I tend to categorise things into 1. Urgent and Important 2. Important but not urgent 3. Urgent but less important 4. Not important and not urgent and do tasks in that order (although I might do a 3 before a 2 if it is a quick and easy one). You should always ensure that anything that affects patient care is categorised as 'important' and otherwise use your judgement/common sense...
In tray exercises are kind of hard to prepare for but I would ensure you think through what kind of tasks/knowledge you would expect to be using in the job (e.g. email management, diary management, people/HR management, finances) and brush up on anything you are less confident on, and then otherwise focus on preparing for the interview section as that typically pays more for solid preparation as you can predict and prepare answers for questions more easily.
Good luck!