Usually, if the threshold is displaced, it is because of obstructions, such as trees, powerlines, or buildings off the end of the runway which prohibit you from making a normal descent and landing on the begining of the runway. Officially, your not supposed to land on them but you can if you want to because that part of the pavement still supports landing aircraft.The displaced part is only used for takeoff or taxi up for takeoff. Also the displaced part does not go into the overall runway lenght. Even if you have a runway that is 5000 feet but has a displaced threshold of 2000, the legal runway lengh for landing is only 3000. That threshold picture above signinficantly reduces the runway landing lenght for runway23.