Plain English Meaning
A red sign with white runway numbers placed beside the taxiway. It tells the pilot, 'Stop here. You are about to enter a runway. Do not go past this point without permission.'
Full Definition
A mandatory instruction sign, displayed as white characters on a red background, that marks the holding position on a taxiway before it enters a runway. The sign shows the runway designation (for example, '15-33') indicating the runway whose protected area begins at that point. An aircraft or vehicle must not cross the associated holding position marking onto the runway without an ATC clearance at a controlled airport.
Context Anchor
Seen while taxiing toward a runway, crossing a runway, or approaching a runway entrance from a taxiway.