Coinsurance is shown as a percentage (such as 20%) and refers to the portion of cost that you are required to pay after you have met your deductible.

For example, if you have a service that cost $2000 and have a deductible of $1000 and a coinsurance of 20%, assuming you have not met any of your deductible yet, you would be responsible for the first $1000 upfront.  After that, you would pay 20% of the remaining $1000, for a total of $1,200 for the service.

As with most things, there are exceptions to this.