A heuristic accessibility evaluation has an experienced accessibility specialist review a product against a broader set of known best practices, common assistive technology behavior patterns, and hard-won lessons from prior audits, in addition to the formal WCAG success criteria, catching usability friction that technically passes WCAG but still creates a poor experience in practice.
It is often used as a lighter-weight complement to a full WCAG conformance audit, particularly early in a design process, when getting expert judgment on likely problem areas is more valuable than a line-by-line criterion checklist against a product that has not yet been fully built.