Mary is a school crossing patrol worker, and the poem watches her at her post: four times a day, five days a week, whatever the weather. Her sign is the center of it. The speaker builds it up like something grand, calling it "a modern sword of Damocles" hanging over the drivers' heads and asking what "strange legend or holy grail" it might hold. The answer is a flat "None." The sign says only "STOP CHILDREN!"
That gap between big language and ordinary reality is the poem's main move, and it's gently funny. Yet the joke has a serious side. Lorries and private cars alike obey, because the sign protects "one hundred souls." The speaker seems to admire Mary and quietly marvel that a simple person with a simple sign can halt everything. The mood is light, wry and warm, with a note of respect for small, steady, unglamorous work that keeps children safe.