The maximum out-of-pocket amount is the most you'll pay for in-network healthcare, excluding monthly premiums, in one calendar year. Every time you spend money on health care (in the form of deductibles, copayments, and coinsurance), it counts towards your maximum out-of-pocket. Once you reach this amount, your insurance company will pay the full cost of any covered medical service as long as you keep paying your premium.
Let's look at an example:
Suppose you require a $100,000 heart transplant and you bought a plan with a $5,000 deductible, 20% coinsurance, and a $10,000 maximum out-of-pocket. Assume you haven't yet contributed anything toward your deductible. You would only pay $10,000 for the surgery rather than your much higher coinsurance amount. This is because you are protected by your maximum out-of-pocket amount.