A sticker auto is an autograph card where the player signed a sticker that was then applied to the card during manufacturing — as opposed to signing directly on the card itself (an on-card auto). The sticker is usually a small, clear or white adhesive label placed in a designated area of the card design.
Sticker autos exist because they're more efficient for manufacturers. Players can sign sheets of stickers at once, and those stickers can be applied to cards later in the production process. It's a logistics play.
Most collectors prefer on-card autos for the aesthetics and the direct connection, so sticker autos typically sell for slightly less. But a sticker auto of a star player is still a premium pull — don't let the format distract you from the substance.