The Small Shield is an item from The Minish Cap.
At the Picori Festival, Princess Zelda will win the Small Shield in a raffle contest. She will give it to Link telling him to protect her. The Shield is used just after it is received to protect Princess Zelda from a Business Scrub in North Hyrule Field.
The Small Shield can block most enemy basic attacks, such as sword strikes or Octorok's rocks. The Small Shield is the most basic shield in this game, and it can be swallowed by Like Likes. Link can defeat the Like Like to retrieve his Shield back, or purchase a new one at the Hyrule Town Shop for 40 Rupees.
Once Link has defeated Vaati, he can trade in the Small Shield for the Mirror Shield. Link needs to complete the Goron Quest and then fuse Kinstones with the Goron on the far right in Goron Cave to make Biggoron appear on top of Veil Falls.
Small Shield Sprite from The Minish Cap