You should expect to tell them about what makes you a strong candidate to study the subject.
Historically, many medical schools have not paid that much attention to personal statements but in the light of potential impact of COVID on applicants they may well be making more use of them in the future.
With interviews, there is often a focus on any work experience the applicant might have had. The interviewers would be wanting the applicant to be reflective and able to speak about what they have learned from the experiences (rather than requiring specific experiences).
Regarding Medicine applications, there is lots of useful advice on the Medical Schools Council website.
The main focus of the personal statement is your relationship with the subject; what is it about the subject that you particularly like, and what is it about you that would make you a particularly strong candidate to study this subject? Much like what David has said about interviews, a good way to demonstrate these things is by mentioning any work experience, plus any wider reading or research you have done, any activities or programmes that are related to the subject, and then analyse and reflect on these experiences to really draw out what you have learnt from them; what new knowledge and skills have you gained that will help you when you come to study the subject at university.
For a more detailed exploration of the UCAS application process watch this video UCL have produced the following video which I would recommend any applicants watch.