It is part of a health insurance policy that pays a pre-determined percentage of your medical costs, sometimes this is after meeting a deductible. There are several variations the most common being 80/20, where the insurance company pays 80% of the bill and you pay 20%. Generally you’ll pay your percentage until you hit a maximum out of pocket, often referenced in plan materials as a MOOP.
For example, let’s say you go to the doctor and he charges $100. Your health insurance would then pay $80 of that charge. The remaining $20 is your out-of-pocket expense.