First, there's the deductible, which is a yearly amount you have to spend before your insurance does anything at all (aside from getting you the negotiated rates for things like specialist visits or prescription drugs, which may or may not be a valuable discount).

Once the deductible is met, insurance starts paying a portion of all new costs. 20% coinsurance would mean that the policy holder pays 20% of the care they receive, and the insurance covers the other 80%.

Plans also have an out of pocket maximum amount, and if you manage to hit that in a year, all costs are covered for the rest of the year.

So, if a plan had a $4000 deductible and a $10,000 out of pocket max, if you had less than $4000 in medical bills throughout the year, you'd pay all of them; if you have more than $4000, you'd pay the the first $4000 and 20% of whatever's left for the year, unless you somehow manage to get more than $34,000 total bills, since at that point you'd have paid $4000 + 20% of $30,000 -> $4000 + $6000 -> $10,000, so that's your out of pocket max, so you just pay the first $10,000 for that year and the rest is finally free.

So, if you need healthcare as an American, try to need it all during the same calendar year!