Shield Statistics
Source Player Core pg. 250
Shields have statistics that follow the same rules as armor: Price, Speed Penalty, and Bulk. Their other statistics are described here.
AC Bonus
Source Player Core pg. 250
A shield grants a circumstance bonus to AC, but only when the shield is raised. This requires using the Raise a Shield action.
Hardness
Source Player Core pg. 250
Whenever a shield takes damage, the amount of damage it takes is reduced by this amount. This number is particularly relevant for shields because of the Shield Block feat. The rules for Hardness appear here.
HP (BT)
Source Player Core pg. 250
This column lists the shield's Hit Points (HP) and Broken Threshold (BT). These measure how much damage the shield can take before it's destroyed (its total HP) and how much it can take before being broken and unusable (its BT). These matter primarily for the Shield Block reaction.