People have different ideas about what the song means but to me it’s about the social movement of the 60’s. That people were worshipping the “things” of the world (neon gods), while people (prophets), were forewarning the dangers of doing this and the social problems that were being neglected; the prophets being street artists because they often address social causes in their murals. Lastly, that silence is like a cancer. That people often don’t speak up unless it’s a cause that affects them and that eventually the cancer will spread and impact them and it could be too late.
Anyhow, some people have different ideas about the song but the above makes the most sense to me. Regardless, the lyrics and music are amazing and of course the perfectly blended voices of Simon & Garfunkel make it better still.