A pop report is the grading company's census — a published count of how many copies of a specific card have been graded at each grade level. Pull up a pop report and you'll see exactly how many PSA 10s, BGS 9.5s, or SGC 10s exist for any given card.
Why does this matter? Scarcity drives value. If a card has a low pop count at high grades, that means fewer pristine copies are out on the market, which typically means higher prices. Conversely, a high pop count means more supply and softer prices.
Smart collectors check the pop report before buying or submitting a card for grading. It tells you whether that GEM Mint 10 is truly rare or if there are hundreds of them floating around. It's one of the most useful tools for making informed decisions in the hobby.