A durometer is a gauge that measures how hard a tire's rubber is, read on the Shore A scale from 0 to 100. Lower numbers mean a softer compound; higher numbers mean harder.
Longacre’s tester “reads from 0 to 100 points on the Shore ‘A’ scale.” The catch is that a durometer is easy to fool. Temperature moves it — “hot tires will be softer than cool tires,” and the side of a tire facing the sun reads softer than the side facing the shade. Technique moves it too: apply the same pressure every time, roll the footpad flat, take the reading fast and get off, because leaving it planted deforms the rubber and gives you a false number. Scrape loose rubber and track pickup off a used tire first. Racers use it to match tires within a set, sanity-check that a compound is what the sidewall says, and track how a tire hardens as it heat-cycles.
Compound is one of the biggest grip variables you control, and a durometer is the only way to check it instead of trusting the sticker.