On each individual staff member's profile under the Settings tab it's possible to set the amount of leave that is carried over each year. At the end of your holiday year the number of days specified is added to the employee's new year's holiday allowance.
So for example, say there's an employee who is given 25 days holiday each year. Let's say that you've set their holiday carry over to 5 days. At the end of the year that employee has 10 days of holiday left, only 5 of those will be carried over in to the new holiday year. So in the next year they will have 30 days' holiday.