In Plato's book "Phaedus", Socrates go further saying "Did we not see equalities of material things, such as pieces of wood and stones, and gather from them the idea of an equality which is different from them?" and he also said "Do not the same pieces of wood or stone appear at one time equal, and at another time unequal?"
What did Socrates mean by that ?
The passage quoted in the question is from Plato's Phaedo 74b.
In the passage around, Socrates aims at the difference between two concrete things, which are equal, on one hand, and on the other hand the abstract idea of equality. He gives an example: Two pieces of wood may seem equal. But in order to recognize that they are are equal, we must have formed before the idea of equality - according to Socrates.
That's one of the many points in Plato's work, where Plato lets Socrates perform Plato's own doctrine. Here the theory of forms in the context of Plato's doctrine of anamnesis, which means to rediscover in our life those ideas which the soul has learned before during its pre-existence.
