These are a set of renamable runway and taxiway signs used at airports.
They appear as Airport Runway Sign and a number in the Scenery menu. The numbering system refers to a list – place a sign and then click on it with the ? menu button to display an HTML page to explain the numbering system and list of signs.
Some allow you to type in runway or taxiway numbers. All are lit up at night, and height adjustable, so you can lower a sign into the ground to make the lamp head less obtrusive.