Ka is the equilibrium constant for an acid. It measures the amount that the acid dissociates into a proton and the conjugate base. The more the acid dissociates, the more protons are produced, and the stronger the acid is. So it essentially is a numerical measure of the strength of an acid. Since most organic compounds are weak acids, this can give very small numbers to work with. So for convenience sake, most chemists use the pKa since it gives easier integers to work with and compare. The mathematical relationship is shown below:
pKa = -log(Ka)
This is similar to the relationship between pH and proton concentration. So the smaller the number, the stronger the acid is. A strong acid like HCl has a pKa of -2. Acetic acid is a weak acid with a pKa of 5. Methanol is very weakly acidic and has a pKa of around 15.