Caster is the fore-and-aft lean of the steering axis — the line through the upper and lower ball joints — viewed from the side, in degrees. Positive caster tips the top of that line rearward.
Caster is what makes the car track straight instead of wandering — JOES Racing calls it “directional stability.” On a left-turn car the useful part is caster split: more positive caster on the right front than the left, so the car wants to fall to the left and the driver isn’t fighting the wheel all night. JOES reports crew chiefs commonly running “3 degrees positive on the right side and 1 degree positive on the left side,” sometimes 4 or 5 right against half a degree left, and less split on rough tracks. Split also moves wedge dynamically — more split de-wedges at full steering input and adds it back as you unwind. RC racers change caster with blocks and clips, and it does the same job.
The right caster split lets the car turn itself into the corner and settle on the straight, so the driver spends the night driving instead of steering.