Tight is when the front tires lose grip before the rear tires, so the car won't turn and drifts up the track no matter how much steering you add. Also called a push, or understeer.
World of Outlaws puts it as “the car is not turning sharp enough, seemingly pushing into the wall.” Like loose, it needs a phase before it’s actionable — tight in, tight center, and tight off are different animals. Standard first moves run opposite to a loose fix: take cross-weight out to free the car up, wake the right-front tire up with a little less air, and on bias-ply add stagger. A push costs more lap time than most drivers think, because a scrubbing front tire bleeds speed the entire way through the corner instead of just at one point. Check the driver too — over-slowing on entry manufactures a tight center.
A tight car scrubs speed through the whole corner, so it's usually the more expensive of the two handling problems.