This belief of Socrates is one of the most difficult for a contemporary person to understand. On the abstract level, Socrates and Plato both believe that what is more real (and, therefore, superior) cannot be adversely affected by what is less real. On the more concrete moral plateau, which is referenced in the Apology, Socrates professes that a bad person (that is, one which is less real) can not harm a better one.
It appears that he had something like the following in mind. True harm involves
remaining in the wretchedness of ignorance and vice. Since each of us is
personally responsible for the state of our soul each can move out of this
wretchedness through our own effort and no one can impose this evil on us!
Sure, others can harm our bodies, even kill us, but this is nothing compared
to the moral pollution we impose on ourselves!