A green track is a surface that has little deposited rubber on it and therefore offers less grip than the same track will later in the day. You'll meet one first thing in the morning, after rain has washed the surface, on a newly laid or rarely used circuit, or after the track has been cleaned.
On a green track the kart slides more easily, braking distances are longer, and you generally can't commit as hard as you will once grip comes up. It rewards patience and smoothness rather than trying to force yesterday's or this afternoon's pace out of it.
The important point is that a green track is a changing condition, not a fixed one. As karts run, they lay a film of rubber along the ideal line, and grip rises over the session toward a rubbered-in surface. Temperature, cleaning, and fresh rain all push it back the other way. Because the surface is moving, a lap time set on a green track isn't directly comparable to one set later — which is why grip is treated as a major confounder in how weather and track conditions affect comparisons.
Adapting your braking points, entry speed, and expectations as the track comes in is a core skill; adapting to grip changes across a race day covers how.
See also: Rubbered-in. Related terms: grip, marbles.