It's a syllabic form, 5-3-5 syllables, that rather looks like a crescent moon on the page. Developed as a sort of protest to "complicated Japanese traditions" as the website definition puts it, it can be on any subject or thought.
As a genuine haiku addict, my original thought when I heard of this form was what's the point? Since I know that it made the 1987 version of The Teachers and Writers Handbook of Poetic Forms, a book I've never read but which I've seen quoted with some bizarre haiku definitions, I suppose this form may have been taught in school to those young enough to be in elementary school in the late 1980's.
Here's my attempt to write one (it'd look more like a crescent moon if the first and third lines were indented but I'm not quite sure how to do that):
night air filling up
I suppress a yawn
Actually describing the exact moment I am experiencing right now - the sound of crickets outside, the sleepyness I feel, this poem is probably more haiku-like than I think this form is really supposed to be, but what can I say? I'm a haiku poet at heart.
Why this discussion of lunes? Today, I received rootedfool's chapbook of lunes, Bramble in the mail. The poems aren't exactly haiku, but many of them nicely satisfy my short-poem appetites. The ones I like the best are the ones that are most haiku-like. There are a couple of them if you read aloud and don't look at the way they're aligned on the page, you'd accept them as haiku. Those poems are the ones I found myself appreciating the most.
For whatever reason, I like contrast in very, very short poetry and prefer it when there are two parts to a whole that play upon each other.