Plain English Meaning
Lines on the wing that show where the strong internal beams run underneath. If you need to step on a wing, these lines tell you where it is safe to put your weight.
Full Definition
Reference lines marked or visualized along the wing that indicate the position of the internal main structural beams (spars) running from the wing root to the wingtip. They identify the strongest load-bearing areas of the wing and are used as guidance for safe walking, stepping, or placing weight on the wing surface during ground handling, fueling, or maintenance.