This is the barometer setting on your altimeter at ground level up to the transition altitude (18000 in the US) it should be set to the actual barometer on the ground. When you go past 18000 ft set it to the standard pressure of 29.92 inches or 1013 hPa. When descending through 18000 ft. find the barometer reading at the airport you are going to land at. If you display the Map (m key) you can find the airport and right click it. Or go to a website where you can search on the Metar for an airport get it from there.