Resonance effect - Effect of substituents on acidity
Same as inductive effect, if the resonance producing group exerts minus effect that is, if it withdraws electrons, it raises the strength of the benzoic acid. Likewise, if the group causes +R affect it decreases the acidity of benzoic acid. Though, remember that resonance effect influences only o- and p- positions. So if resonance producing group is exist in the m-position it will not apply its effect.