"My voice is my improvisational instrument, the melody instrument. The guitar is harmonic structure. I'm not a good enough guitarist to improvise on it"

Paul Simon is revealing his musical skills and how he utilizes them to produce music. He is saying that his voice is his primary instrument for improvisation, as he can utilize it to develop tunes. He is likewise saying that he utilizes the guitar to provide harmonic structure, but he is not a good enough guitar player to improvise on it. This quote shows that Paul Simon is a skilled musician who has the ability to use his voice to create melodies and his guitar to provide harmonic structure. He is also simple sufficient to admit that he is not a good enough guitar player to improvise on it. This quote reveals that Paul Simon is a gifted musician who is able to use his voice and guitar to produce music. He is also simple adequate to confess that he is not a master of the guitar and wants to learn and improve.