pKaH is a guide to leaving group ability

pK_aH refers to the pK_a of a leaving group's conjugate acid. The lower the value, the better a leaving group it is:

The leaving group ability of an anion depends on how stable it is at carrying negative charge. While pK_a is the negative of a molecule's ability to dissociate into a proton and an anion. So this relation should also make sense intuitively.

Remember this is only a guide. For example, it does not factor in the stability of the carbonyl it creates by leaving.

The above table explains which group is pushed out in the attack below: