A certified autograph is an autographed card that's been authenticated and guaranteed by the manufacturer — meaning the card company itself vouches that the signature is real. You'll typically see "Certified Autograph Issue" printed on the card.
Certified autos carry built-in trust because the manufacturer handled the entire signing process — from getting the player to sign to inserting the card into packs. There's no question about authenticity, which is a huge deal in a hobby where forgeries exist.
This distinguishes certified autos from "cut signatures" or aftermarket-signed cards, which may require separate third-party authentication. If you want maximum confidence in an autograph's legitimacy, certified is the safest route.