The Deku Shield is Link's first and weakest shield in The Legend of Zelda: Ocarina of Time. It can only be used by Link as a child, and is the only shield that Link can use correctly as a child.
The Deku Shield is needed in the very first minutes of the game in order to get past Mido, a Kokiri blocking Link's path to the Great Deku Tree. It can be bought from the Kokiri Shop for 40 rupees.
Small and wooden, the Deku Shield has a swirl design carved into it that resembles the Spiritual Stone of the Forest. It is very weak against fire and will burn to ash instantly if Link touches a lit torch, lava, or something similar and fiery. Because of this, the larger and sturdier metal Hylian Shield can be more dependable in certain situations, but child Link can't actually use it like a shield in Ocarina of Time (it's too large), so the Deku Shield is generally a better choice.