"Wabi" is simplicity.  "Sabi" the beauty of age and wear.  It is the Japanese thought rooted in Zen Buddhism of finding beauty in every aspect of imperfection whether man made or in nature.  It is the aesthetic of things in existence that are modest and humble, imperfect, impermanent and incomplete.  It is the acceptance and appreciation for the evanescence of life - "for everything there is a season."