This is the classic "argument by analogy" and is commonly found in Platonic dialogues. It always compares two things that are dissimilar on the surface but, nonetheless, similar in some key way.
Here, Socrates compares athletes who care about
improving their athletic performance with those who care about the improvement
of the soul. Just as the athlete must listen to the trainer (and not
to the careless statements of those who lack a knowledge of athletics), he
and Crito must only listen to those who are
knowledgeable about the issues at hand, namely justice, fairness, and the