Plain English Meaning
Rows of white lights laid out in a line on the ground leading up to the start of the runway. Pilots use them to find and line up with the runway when breaking out of cloud or low visibility.
Full Definition
The series of steady white light bars that extend along the centerline of an Approach Lighting System (ALS) outward from the runway threshold, in the direction opposite to the landing aircraft's approach path. They form the lighted lead-in line that pilots follow visually toward the runway during the final segment of an instrument approach.