The Deku Leaf is an item found in The Wind Waker. Its primary use is that of flotation, allowing Link to fly from platform to platform with ease. Link receives this item from the Great Deku Tree, as Link requires this item if he were to try to save Makar from the Forbidden Woods. Once Link uses it, he can jump off the platform he's standing on, and Link will float to another spot. The Magic Meter comes with it. When this runs out, one will no longer be able to use the Deku Leaf, and will simply be left with a broken leaf. One can cut down some grass to get some more magic.
The Deku Leaf has one more use. Link can use it to create a gust of air when standing still. He can use this gust to move certain objects, allowing Link to access new places.
Deku Leaf from Cadence of Hyrule