What it is
Looks up the domain's position in the Tranco list, which combines rankings from Umbrella, Majestic, Quantcast, the Chrome User Experience Report and Cloudflare Radar into one deliberately stable top-million list. Sites outside that million have no rank.
Why it matters
A rough sense of scale, most useful for comparing domains rather than as a number in isolation. Tranco averages over time, so it moves slowly, which is the point: it resists the day-to-day churn that made earlier lists unusable for research.