Okay, I have struggled with this remark in the past and I continue here. Wittgenstein asks us to think about a comparison between "knowing" and "saying." He offers a series of statements or displays of knowing. Wittgenstein says, for example, that "how many feet high Mount Blanc is" is an illustration of knowing and not being able to say it. But if I knew how high Mount Blanc is, I would be able to say it. "Mount Blanc is this many feet high." There is a difference between this knowing that can be expressed in a statement and the knowing involved in knowing "how a clarinet sounds." This would be an instance of ostensive knowing. If I could identify the sound of a clarinet -- "that is a clarinet!" -- this would verify my claim to know the sound. In the Mount Blanc example, the verification is contained in the claim. Either I have the measurement correctly or I do not. Ah me. This is a hard one.
Christopher Robinson & Joseph Duemer read Wittgenstein's Philosophical Investigations