The difference between being a tight car or a loose car on track: A car is hard to turn when it’s tight because front tires lose traction more quickly than the rear. Drivers often have to stop accelerating while turning to steer more easily. A loose car, when the rear tires lose traction first, tends to fishtail because it turns too easily.
In a tight car (understeer), the car has trouble steering sharply and smoothly through the turns as the front end pushes toward the wall.
In a loose car (oversteer), the rear tires have trouble sticking in the corners and leads to the car "fishtailing" as the rear end swings outward in the corners.
How does this get adjusted? The diagram below breaks it down. (Key for below: Ratchet inserted by crewman (1), Ratchet extension (2), Side window (3), Rear window (4), Screw jack (5), Chassis frame (6), Coil spring (7), Trailing arm (8), Trailing arm end (9) and Goodyear tire (10).
During a pit stop, one crewman will sometimes add or subtract spring pressure on the tire by attaching a ratchet (1) and manually rotating it one way or the other. This tightens or loosens the spring (7) and also brings the frame (6) and trailing arm (8) of the tire (10) forward or away from each other, while applying more or less pressure on the tire when it goes into a turn. This is more commonly known as adding or subtracting a wedge.
In addition, track temperature and tire wear are both important factors in how the car handles throughout a race.