This will make your ships on your own traderoutes always leave at least X tonnes of the specified ware at the island (so there's always some stock left to be used for citizen needs/production). At the bottom of that screen should be an option reading something like 'Minimum Stock: X t' (sorry, I'm playing in german, so I'm not sure about the exact wording).
In the warehouse-interface (right?)-click the offending ware, where you can set buy/sell-orders.
