To know the relevance of something is to know why it matters or how it is important. I don't understand the relevance of this discussion: it doesn't seem important to me.
If you don't understand the relevance of the word "relevant" to the discussion of the meaning of the word relevance, we're gonna have some trouble. When something is "relevant," it matters. Its relevance is clear. Relevance is simply the noun form of the adjective "relevant," which means "important to the matter at hand." Artists and politicians are always worried about their relevance. If they are no longer relevant, they may not keep their job. Someone without relevance might be called "irrelevant."