It's a Japanese phrase that very roughly means "the 'ahness' of things." It's what Haiku tries to catch, but is more a state of brief poignant awareness of the realness of things very transient, and the transience of things very real.
I felt its touch this week, today, when I got home from a day long meeting and realized that even in the cool days of early spring a tree hanging over my driveway had flowered and lost its moon shaped tiny white petals in the space of 48 hours. They are in the crevices of my driveway now, just a few left because the wind blew most of them away.
When I got home I saw the 'tulip tree' that also hangs over my driveway, which I call that because its flowers are enormous tulip shaped things with magenta to white petals. Most years it blossoms one day and a rain storm washes them down the next.
But this year it has been dry and sunny even if cool. The tree blossomed Thursday and is still full of flowers today.
Muno no aware.