What's an MMI interview and how can I prepare for one?

MMI stands for Multiple Mini Interview. Unlike a traditional panel interview where 3 or 4 staff members grill you for 20-30 minutes, MMIs are broken down into roughly 8 "stations" lasting ~5 minutes each. They might ask you to read a newspaper article and critically analyse it. They might ask you to tell them about parts of your personal statement. They might present an ethical dilemma and discuss it with you. The best way to prepare for these is to go on the university's website and read absolutely everything they have written about their interviews and what to expect.
Then, go on The Student Room and read the threads discussing the interviews from the previous year. For example, if you were applying to Leeds I would google "Leeds medicine interviews the student room" and see what people are saying. Beware though, some people deliberately make things seem harder/easier than they were in order to put people off. Remember that this is a competitive environment and everything that is not directly from the university must be taken with a grain of salt.
These kind of interviews are trying to find the "real" you. Just be honest, think out loud and try and make some intelligent suggestions. Often there is no correct answer. Good luck!