What is the relation between pH and pOH?

pH = pKa + log([A-]/[HA])

pH = pKa+log([conjugate base]/[undissociated acid])

The actual meaning of pKa: the negative log of the dissociation constant, which is a measure of strength of an acid/base

when pKa = pH, there is equal concentration of acid and its conjugate base. pKa helps to understand the nature of acid and base like pH:

pKa 2 but 7 but < 10 -- weak base pKa >10 --strong base