What is the operational function of Takeoff Hold Lights (THL) as part of a Runway Status Lights (RWSL) system?

1. They light up red along the runway centerline to warn a pilot on the takeoff roll that another aircraft or vehicle is on, or entering, the runway ahead.
2. They provide green centerline guidance lights to assist pilots with maintaining alignment during high-speed takeoffs.
3. They indicate that the aircraft has been cleared for takeoff and the runway is clear.
4. They illuminate when wind shear is detected on the runway surface.