Stagger is the difference in circumference between the right- and left-side tires on the same end of the car. A bigger right-side tire makes the car want to turn left on its own.
Measure it with a flexible stagger tape around the center of the tread, at race pressure, on both tires. Longacre’s example: “if your right rear tire is 86-1/2” in circumference and your left rear tire is 84”, then you have 2-1/2” of stagger.” How much you want isn’t a fixed number — Longacre ties it to “the radius of the corners and the amount of banking,” so a tight bullring and a half-mile want different answers. On bias-ply tires you can build or bleed stagger with air pressure; radials barely respond that way, so you’re picking sizes instead. Stagger also walks around as tires wear and heat up, which is why you re-measure every session rather than trusting last week’s number.
Too little stagger and the car fights you through the middle; too much and it's trying to turn left all the way down the straightaway.