I love this quote, so off-hand, so glib, and often, so true.
Art is a process of trying and failing. You see something in your head and try to make it live on the page--or on the stage; or you feel a tune in your body and try to coax it to come through your instrument. Often what seemed like a good idea doesn't emerge half as nicely as you conceived it, but as you play with your concept you begin to notice a part of it that really takes your imagination by surprise and you're OFF, pursuing that glowing idea that hid within the earlier impulse.
Art is not so much knowing what "mistakes" to keep, but pushing through the mistakes to find that living seed with the potential to become something beautiful.