The Magnitude of Ka or Kb.
HA + H2O ¾ H3O+ + A-
Ka represents how far this equilibrium proceeds. That is Ka is a measure of how stable the anion A- is in water.
Anions which can stabilize the negative charge will be more stable than those which cannot.
Compare NO2- with NO3- for example:
Nitric acid is a strong acid (fully reacted) whereas nitrous acid is a weak acid.
Here the stability of the anions is due to delocalization (resonance) of the electrons; the more delocalized (spread out) the charge is, the more stable is the ion.
Another type of stabilization of an anion that can be seen by looking at pKa values is that for substituted acetic (ethanoic) acids:
(if pKa decreases, Ka is increasing)