An 'Angel' is a term for when a highly respected or famous graffiti artist passes away. The people who admire an angel will tag the name of said angel putting a halo over that tag. Some will also make pieces, or even throw-ups as a tribute to the late graffiti artist, and will go to certain pieces left by an angel to mourn.
Going over, or dissing an angel's work (or even the tribute pieces dedicated to that angel) will not only draw extreme ire from artist society and will make you a cast, but it's also seen as disrespectful to the dead and will completely destroy all your rep, not just for the city but the entire graffiti community. Therefore, an Angel's work must be respected at all costs