An Auto-Jump is performed whenever Link approaches an unobstructed ledge with enough speed. Rather than simply fall off the ledge, he will automatically leap from it, allowing him to cross short gaps. This occurs in every 3D Zelda title, with Ocarina of Time credited with the initial innovation.
By making the jumping action automatic, an extra button in the control scheme can be allocated to other actions such as using items. Barring special items such as Roc's Feather and Roc's Cape, The Adventure of Link is the only main Zelda game to have a button dedicated to jumping. In 2D games, Link will automatically jump (and often flip) off ledges, but cannot cross gaps without the aid of items.