Passive shield
While all characters have access to a bubble shield, Link, Young Link and Toon Link carry their own personal shield while in battle. While standing still, incoming projectiles that hit the shield will deal no damage and simply push Link back, dependent on how much damage and knockback it would ordinarily do. This effect was first implemented in Melee and has been retained in all installments of the series since then. Link's shield in Super Smash Bros. 4 was altered slightly - Link now takes a defensive stance when hit, and his shield is active while he is walking.
Since projectiles must hit the front of the shield to be blocked, Link may still be hit in the back, head, or feet, and some idle poses can cause Link to move the shield away from the front of his body. Crouching will cover Link's legs and disable the idle poses, but then any projectiles that would have hit the shield normally may instead hit Link's face. Link's shield is unusable when he is holding an item.
To be considered a projectile for the purposes of the shield, a hitbox must be reflectable and not unblockable.
While Link wields a shield in Super Smash Bros., it only exists for aesthetic purposes, and has no effect on gameplay.
Origin
Link has always possessed a shield in every Zelda game to defend himself against attacks. Depending on the game, it may be an always-active ability so long as Link is standing or walking, or it may have to be raised manually; in either case Link must be facing the attack in order to block it. In some cases, powerful attacks are too much for the shield and may cause Link to stagger or damage him anyway.
The design of each Link's shield is taken from the respective game. Link carries the Hylian Shield from Ocarina of Time in both Super Smash Bros. and Super Smash Bros. Melee while he uses the shield's Twilight Princess design in Super Smash Bros. Brawl and Super Smash Bros. 4. Young Link uses the Deku Shield from Ocarina of Time (which does not burn against fire like its original counterpart), while Toon Link holds the Hero's Shield from The Wind Waker.
Link being capable of shifting his shield's position by crouching is reminiscent of Zelda II, where Link can either defend his upper body or lower body by crouching and uncrouching. In addition, one-handed swords in Ocarina of Time and Majora's Mask would cause Link to crouch if he shielded without Z-Targetting.
