Green peppercorns are the same unripe berries that come from the same vine as black peppercorns (piper nigrum). They are simply the same black peppercorns, only picked early and unripe. As a result they are much milder than black or white peppercorns and have more of a vegetal and fresh flavour. They are often used fresh in Thai dishes or French dishes such as the French steak au poivre where the whole green peppercorns are in the sauce. They are generally dried or can be found fresh in brine given they are perishable. You can place these peppercorns into water to rehydrate them for use in cooking should you wish to do so. They make a great addition to many dishes given their vegetal properties.