Spring rate is a spring's stiffness — the force it takes to compress it one inch, written in pounds per inch. A 400 lb/in spring needs 400 pounds to move one inch, 800 pounds to move two.
Rate is stiffness, not load — the two get mixed up constantly. A linear spring’s rate doesn’t change with how hard you squeeze it; QA1 notes it “is not affected by the load that is put on the spring.” That’s why two different rates can hold the same corner weight at different amounts of compression, and why smash numbers exist. The number stamped on the spring also isn’t what the tire feels: Penske puts wheel rate at spring rate times motion ratio squared, so a 400 lb/in spring at a 0.7 motion ratio is about 196 lb/in at the wheel. RC racers tune the same variable — and there, too, preload sets ride height, not rate.
Spring rate decides how much the car moves under load, so getting it wrong makes every other adjustment you try read wrong too.