Effect: This property provides an enhancement bonus to confirm critical hits and critical hit damage. A +10 Seeker item adds +10 to your confirm critical roll and +10 damage to your critical damage (before multipliers).
The additional damage only applies if you actually score critical damage. If your target negates the critical hit, for example by fortification, extra seeker damage does not apply.
Seeker affects all your attacks (single weapon melee, dual weapon melee, and ranged). For example, if you are using two weapons, only one needs to have seeker but both benefit from it. Or you can place seeker on an accessory. The same is true of Backstabbing, Doublestrike, and Stunning.
Seeker items can provide various types of bonus. Regular Seeker grants Enhancement bonus, then there's Insightful Seeker and much rarer Quality Seeker. The former Exceptional Seeker grants Insight bonus.