The scale, from slowest to fastest
Heavy is the slowest: the ground holds every hoofprint and stamina becomes everything. Soft is testing but honest. Good to soft leans easy. Good is the ideal most courses aim for. Good to firm is quick ground where speed horses shine. Firm is rare in Britain and Ireland now, because courses water to avoid it; when it appears, trainers of soft-ground horses stay home.
Who decides it, and how
The clerk of the course walks the track and publishes the official going, backed by a device called a GoingStick that measures how much energy the ground absorbs. Readings and descriptions are published before declarations, then updated on raceday if weather intervenes. Irish courses use the same language with a local accent: you will hear "yielding" in Ireland where Britain says good to soft.
Why punters care more than anyone admits
Form reads differently once you know the ground it was earned on. A horse beaten twenty lengths on heavy may be a different animal on good; a course-and-distance winner on firm tells you little about today's bog. When our stats pages show a trainer's record, the going filter is where quiet edges live.
The phrase you already know
"The going is good" crossed into everyday English from exactly this. On a racecourse it is simply the forecast, and the forecast decides who runs.