A Grace Period is an optional amount of time that you can add to the end of a license. This means that the user will be able to continue to use your product for an amount of time you choose, even after the license has expired. This can provide a small amount of time for a user to complete a missed payment which can be useful for retaining users.
A grace period can be set to be any, whole number, of days. This can be set through the get license capabilities end point. When you do this it adds the number of days to the returned end dates.