The valuation cap is the ceiling at which a convertible instrument converts into equity. If the next priced round happens at a higher valuation than the cap, the early investor still converts at the cap price, getting more equity than the cap math would suggest. If the round happens at or below the cap, the cap is irrelevant and the discount (if any) governs.
Caps are the most negotiated number in early-stage SAFE and note rounds because they directly determine the ownership split between founders and early investors when the next priced round happens.