If you set products that are for sale with quantity of -1 then it will be available regardless if you have stock or not - this is ideal for items that you order from a supplier on demand.
If you set product quantities > 0 then they will reduce on each sale until it reaches zero at that time it becomes unavailable for purchase until you increase the quantity.
If you set product quantities > 0 then they will reduce on each sale until it reaches zero at that time it becomes unavailable for purchase until you increase the quantity.