When Westerners first began to visit Japan in the mid-1500s they were struck by the refined beauty and quality of the country's arts and crafts. It was a kind of beauty and quality that they had never seen before.
This special quality of Japanese things was so commonplace that the Japanese themselves did not consider it unusual. Everything they made, including simple household utensils, had the same quality.
Quoting from my book The Elements of Japanese Design:
Yugen beauty referred to a type of attractiveness beneath the surface of the material but in delicate harmony with it that registers on the conscious as well as the subconscious of the viewer. It radiates a kind of spiritual essence.
For a definitive look at the Japanese view and creation of yugen beauty, see Elements of Japanese Design - Key Terms for Understanding & Using Japan's Classic Wabi-Sabi-Shibui Concepts.