This quote means that everyone has different talents, abilities, and strengths. It is not fair to judge someone by what they cannot do. As an example, a fish trying to climb a tree will never be able to do it, and so if we judge it by that, we’re not taking into account any of its other talents or qualities. The fish in this case will then spend its life thinking it is less than it actually is, believing that it isn’t smart or capable, when that isn’t the case.