https://www.railexpress.com.au/testing-of-turn-up-and-go-metro-tunnel-services-underway/
The Metro Tunnel has begun testing turn-up-and-go services along the new lines, using its state-of-the-art signalling systems.
It means there's less than a 10 minute frequency between trains, i.e when you want or need to catch a train, you don't look at the timetable and try and time it so you arrive not long before your train is due - you just turn up and go.
I believe globally it's usually a 5 minute frequency which would really be turn up and go, but here they use TUaG to mean a 10 minute frequency. Personally, I wouldn't really consider 10 minutes to be turn up and go, I'd still try and time things so I arrive 2 or 3 minutes before the train, but it's a step up from the former 20 minute frequencies.