No, it isn't something you put on sushi. Wabi-sabi is a Japanese philosophy relating to concepts of beauty and grace in culture and nature. Traced back to writings in the 13th century, and with many links to Shinto Buddhism, it has been described as the beauty of things "imperfect, impermanent and incomplete."
Wabi-sabi comprises many ideas. Wabi means transient and stark beauty and sabi refers to the beauty of natural patina and aging. This is linked to the concept of transience which is fundamental to the Buddhist philosophy. Wabi-Sabi cherishes beginnings and endings, and the evidence of changes that occur with the passing of time. It prefers to show a flower in bud or in decay rather than in full bloom. It cherishes the dent in a table, the wrinkles on a hand or the crack in a vase.
Zen philosophy is also linked to the concept of wabi-sabi. The primary Zen aesthetic principles listed below form the basis for Japanese garden design, famous the world over for it's beauty and serenity. They are also seen as virtues of human character to be cultivated.
Fukinsei: asymmetry, irregularity;
Koko: basic, weathered;
Shizen: without pretense, natural;
Yugen: subtly profound grace, not obvious;
Datsuzoku: unbounded by convention, free;
Seijaku: tranquility, stillness.
Sounds lovely, doesn't it?
The concept of Yugen is one that I had come across separately. It has been described as "an awareness of the universe that triggers emotional responses too deep and powerful for words." I don't know about you but that is exactly how natural beauty and great art affect me. These profound emotional experiences allow us to transcend the trivial and mundane, and tap into something greater than ourselves, evoking a sense of humility and wonder.
But what I like most about wabi-sabi is the celebration of imperfection. Got a tumble-down fence? Mismatched china? Dust accumulating on the window sills? Just nod your head with an inscrutable smile and murmur "ah yes, wabi-sabi."
If you type yugen or wabi-sabi into a search engine you will find plenty of fascinating articles and images related to the topic. I've added one link to a garden site that explores the Zen aesthetic principles.