A rubbered-in track is one that has had substantial tire rubber laid down along the racing line over a day or a weekend, giving noticeably more grip than the same surface offered when it was green. It's the condition you'll usually find later in a busy race day, on the main line where the karts have been running.
With more grip you can brake later and carry more commitment, and the kart tends to feel more planted. Two cautions come with that, though. The grip lives mainly on the line — step off it, onto the dusty or marble-strewn part of the track, and grip drops away sharply. And a heavily rubbered surface can become sensitive: very high track temperatures can make the rubber greasy, and fresh rain over a rubbered track can be especially slippery until it washes clean.
Because the grip level has moved, the line, braking points, and even the setup that worked on a green track often need revising once the track rubbers in — a per-track baseline is a moving target, which is why building a baseline setup for each track treats conditions as part of the baseline. Adjusting your driving as this happens is covered in adapting to grip changes across a race day.
See also: Green track. Related terms: grip, marbles.