In a world without insurance, you'd go to a doctor when you had a problem and they'd fix you and send you a bill, which you'd pay in full.

Insurance adds a layer to that, where you pay them a fixed amount every month and in turn they pay the doctor's bills.

Rather than paying 100% of everything, the insurance companies have certain rules – you have a deductible that you must meet yourself first, they will only cover certain procedures, they will only cover a max amount or percentage for each procedure and you must pay the rest yourself, and more.

Now, insurance companies also have tie-ups with certain doctors and hospitals. These doctors will have special rates just for your insurance provider, and so your insurance will cover more of the cost than normal. These are called in-network doctors.

This doesn't mean that out of network doctors won't be covered at all, but your insurance will just have stricter rules for them.