pH = a measure of how
concentrated in hydrogen ions a particular acid is (logarithmic scale) and
does not provide a measure of an acid's strength.
For example, an acid with a pH of 3 will have a hydrogen ion concentration of 10^(-3) mol/L, or 0.001 mol/L (using the fact that log(hydrogen ion concentration) = pH)
Degree of ionisation = how much of available hydrogen ions present in the acid is
actually ionised in solution, and thus
does provide a measure if the strength of an acid (and does not provide a measure of concentration).
Higher degree of ionisation = higher strength of acid
Hope this helps! Good luck with your studies!