Physical Defense in Dark Souls 3 defines the protection your character has against physical damage. It does not protect the character from damage recieved by Magic, Fire, Lightning, and Dark attacks. Physical defense is further subdivided into 4 different types: [ Standard | Strike | Slash | Thrust ]. Many weapons infused with [ Heavy Gem | Sharp Gem | Refined Gem | Raw Gem ] generally deal physical damage and thus Physical defense protects the character from such infused weapons.
Physical Defense is increased strongly by leveling Vitality, which also increases the character's Equip Load. This also allows the character to wear heavy armor which grants additional protection by granting high damage absorption values (which is a separate statistic). Physical defense can also be increased moderately by leveling Strength, although this is less efficient than Vitality (75%). All other attributes also grant Physical defense when leveling up, although at a strongly reduced efficiency when compared to Vitality (33%).