Psi-Strength cannot be improved. Psi-Skill starts low and improves with use or training in the lab.
Your ability to resist alien Psi attacks is almost totally based on Psi-Strength.
Your ability to perform Psi attacks is based on your Psi-Strength multiplied by your Psi-Skill, so a soldier with 50 and 50 is much stronger than one with 80 and 20.
Since Psi-Strength cannot be improved, some Commanders believe that soldiers with low Psi-Strength should be identified and fired as soon as possible, before they are mind-controlled and shoot you in the back (and before you waste a lot of effort training them).