Shield is a status that blocks damage equal to its value. The effect lasts until the shield is removed via damage.
However, damage from Poison will always deal damage to the fighter's health, regardless of shield. Also, certain equipment, like Shadow Dice and Nail Bat as well as equipment which directly reduce health like Rotten Core and Backfire, will directly damage the user even if they are shielded.
In the Parallel Universe episodes, Shield is changed to Shield?. Shield? does not prevent damage from any sources. However, after the enemy's turn, the user's health is recovered by a value equal to the amount of Shield? the user has. This effect lasts until the beginning of the user's next turn.
Equipment like Charge! and Shield Bash do damage based on the amount of shield the user has.
The gadget Barricade grants 3 shield.
The gadget Power Up has a chance to grant 4 shield.