"You know" can be included in the so-called discourse particles, that is those linguistic units of spoken language which have no real meaning, their function being merely a pragmatic one. According to Wikipedia, they serve "to indicate the speaker's attitude, or to structure their relationship to other participants in a conversation."
As a personal note, I add that discourse particles, which are common in speech of every language in the world, may also reveal something about the mindset of a nation. E.g. English people are concerned about other people's understanding of what they say. Italians use "cioè," which means "that is," so they are perhaps more focused on finding a better way to say something or explaining their words.