Extras are either paid with the booking through Clickstay, or paid locally, usually on arrival at the property. The details of when the extra will be paid can be found on the property page and is shown on the cost breakdown on the booking pages.
Compulsory extras: these are charges that owners may add to their property which apply to every booking. These often include things like cleaning, and may be based on the number of people in your group, or length of stay.
Optional extras: these are optional charges that owners may add to their property for you to decide whether you would like to add them to your booking. These often include things like pool heating, airport transfers, a cot for your baby, etc.