Coinsurance is the amount you owe after your health insurance pays its portion of any service, outside of the deductible, usually a fraction of the total.
Example: Chelsea just got an MRI. Her deductible was already met, so instead of being charged the total amount, she pays coinsurance. Her 30% coinsurance rate means that she'll pay $975 instead of the entire $3,250 price tag. This amount goes toward her out-of-pocket maximum, which is the greatest amount she'll have to pay for medical expenses this year.