The Magnitude of Ka or Kb.

HA + H2O 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:

acid pKa
CH3COOH 4.76
BrCH2COOH 2.86
ClCH2COOH 2.81

(if pKa decreases, Ka is increasing)

In this case it is the polarization of the C-X bond that is spreading out the charge from the anion. This effect is called the Inductive effect, and is indirect and so is not as strong as the resonance effect.