The Silver Scale is an aquatic item obtained in Ocarina of Time. It allows Link to dive 6 meters underwater before needing air. Without this item, Link would only be able to dive a total of 3 meters before needing air. It can only be obtained after completing the Diving Game in Zora's Domain. If Link completes it successfully, he'll be rewarded with the Silver Scale.
The Silver Scale is required in order to retrieve Ruto's Letter which is underneath the waters of Lake Hylia. The Silver Scale can later be replaced with the Golden Scale. The Golden Scale allows Link to dive a total of 9 meters before needing air.