Loose is when the rear tires lose grip before the front tires, so the back end slides out and the car rotates more than the driver asked for. Racers also call it oversteer.
“Loose” by itself isn’t a diagnosis — name the corner phase. Loose in (entry, off throttle), loose center, and loose off (exit, on throttle) are three different problems with three different fixes. World of Outlaws describes the symptom as the rear tires having “trouble sticking in the corners, causing the car to fishtail as the rear end swings outward.” The usual first moves are adding cross-weight, adding rear grip with a little less rear tire pressure, or calming down throttle application. Be honest about the last one — plenty of loose-off complaints are the driver, not the car. And a car that only goes loose late in the feature is often reporting on the racetrack, not your setup.
A car that's a hair loose can rotate and drive off the corner; a car that's genuinely loose makes you slow down to survive it.