Plain English Meaning

It's the fixed pressure number every pilot dials into the altimeter once they climb high enough. Using the same number means every aircraft up there is measuring altitude from the same reference, so they all stay properly separated from each other.

Full Definition

The standard atmospheric pressure value used as the universal altimeter setting for flight at or above 18,000 feet MSL in the United States. When pilots set 29.92 in the altimeter's Kollsman window, the instrument displays pressure altitude rather than altitude above mean sea level, and the reading is referred to as a (e.g., FL250 for 25,000 feet).