In Chaucer's "The Canterbury Tales" the reader is introduced to the Pardoner in "the Pardoner's Tale". What is ironic about the Pardoner is that he would often preach that money was the root of all evil, but then he would sell pardons (official documents that pardoned sins).
Portraits are being compared to a beautiful, elegant sunset.
The speaker is saying that a portrait is much more fine and beautiful than other faces one sees throughout the day, and a typical evening's sunset is, similarly, not as beautiful as a shiny, smooth-looking sunshine.