Copays are a flat fixed fee a policyholder pays for a covered service. For example, if a policy has a $25 copay for a doctor's visit, that's all the policyholders would pay for that service, no matter the total visit cost. That gives policyholders clarity into the out-of-pocket costs they pay for a service.
On the other hand, coinsurance is a percentage of the total cost of a covered medical service. The out-of-pocket cost will vary by service and provider. For example, if a policyholder has a 20% coinsurance, they'd pay 20% of a covered medical expense. So, if they had to visit a specialist that cost $300, they'd pay $60 for that visit.