You should know the following:
1) There are two ways of establishing universal healthcare. First is a SINGLE PAYOR SYSTEM. This is when one party, typically the government, pays for peoples' healthcare with tax dollars. This system is seen in many areas of Europe, such as Britain's NHS. The second is an INDIVIDUAL MANDATE, where each person is required to purchase health insurance, and there are usually affordable plans out there. The Affordable Care Act (ACA/ Obamacare) is an example of an individual mandate.
2) Read up on the major provisions of the ACA. What do you think are the best parts of it? Where do you think it didn't go far enough.
3) Know why healthcare is a big issue in the USA (lack of transparency, unaffordable prices, lack of insurance oversight, excessive litigation, etc etc).
4) Know about the effects of each system on Physicians, and some examples of how they've played out in other countries.
Most likely, you won't be asked a lot about healthcare. I've been to 8 interviews and was never asked about healthcare policy, but I know many who have been. You want to make sure you can talk about it if asked. They don't expect you to be an expert, they just want to know that you've thought carefully about the important issues facing the medical profession, of which the healthcare system is a big one.