Plain English Meaning
A pattern of yellow lines painted across a taxiway showing where you must stop and wait before entering a runway. You only cross it when the controller clears you to.
Full Definition
A painted surface marking on a taxiway consisting of four yellow lines—two solid and two dashed—running across the taxiway perpendicular to its centerline, identifying the point beyond which an aircraft must not proceed onto a runway without ATC clearance. The solid lines are on the side from which an aircraft must hold; the dashed lines are on the runway side, indicating an aircraft exiting the runway may cross from that direction.
Context Anchor
You see this marking while taxiing near a runway, especially at taxiway entrances to runways and at runway crossing points.
Why Pilots Care
Clearly marks the limit to prevent runway incursions and ensure from .