A dinged card is a damaged card — and it's every collector's least favorite word. A "ding" can mean indentations, surface scratches, or other types of damage, but the most common culprit is corner damage. When a corner gets bent, blunted, or softened, that's a dinged corner.
Most dings happen from mishandling, but sometimes cards shift around in the pack and come out of the wrapper already dinged. It's frustrating, but it happens. The bottom line: dinged cards get lower grades and lower prices on the secondary market. If you're buying raw cards, always inspect those corners first.