In the early dialogues, Socrates is often shown in the company of someone who professes to know something Socrates himself claims not to know. One of the therapeutic functions of the Socratic method is to bring out the fact that many pretend to knowledge they do not in fact have. Removing a false claim to know clears the way for a humbled pursuit of knowledge.
One who says he knows what virtue is will often reply, when prodded by Socrates, to give instances of virtue. What is virtue? Well, courage is a virtue and so is temperance. And Socrates will reply that he did not ask what things we say are virtues. He wanted to know what these things possess that leads us to call them virtues. In short, Socrates wants a definition, not a list of things to which an undefined term applies.
This insistence (that knowing what a thing is not simply to be able to mention other things that share a name with it) is not made only when it is a question of types of a thing. If for example, in reply to the question "What is virtue?" I should answer, "Well, bravery is a virtue" and Socrates would continue, asking "What is courage?" and I replied by giving examples of courageous acts, he would be equally dissatisfied. What do these instances have that leads us to call them acts of bravery?
We can recognize the individual things around us and notice that some of them share a common term or name. There are lots of acorns on the lawn. What is an acorn? One of those things…