Guard Boost is a stat in Elden Ring.
It decreases the damage to Stamina received when guarding by an equivalent percentage, i.e. a +0 Dagger with its 15 Guard Boost would reduce 100 Stamina damage to 100*(1-0.15)=85 Stamina damage when guarding while a +0 Brass Shield with its 56 Guard Boost would reduce 100 Stamina damage to 100*(1-0.56)=44 Stamina damage when guarding. For 200 base Stamina damage the equivalent numbers would be 200*(1-0.15)=170 Stamina damage for the Dagger and 200*(1-0.56)=88 Stamina damage for the Brass Shield.