Delocalization of electrons decreases charge density, increasing stability. An acid with a conjugate base that has delocalized electrons due to resonance is more acidic than an acid with a conjugate base with localized electrons.

Does delocalization increase basicity?

This delocalization increases the stability of the base. Greater stability results in lower reactivity. A base that has resonance delocalization of the electron pair that is shared with the proton will therefore be less basic than a base without this feature.

Does stability increase acidity?

Since an acid is becoming more negative upon loss of a proton, the stability of the new lone pair on the conjugate base is a key factor in determining how favorable the reaction will be. In other words: any factor which stabilizes the conjugate base will increase the acidity.