It is a Japanese form of poetry with three lines where the first line has 5 syllables, the second has 7 and the third has 5 again. Traditionally they need to have 17 syllables total, but they can also be done in a 3, 6, 3 form as well. They also need to have a seasonal reference and a form of punctuation that juxtaposes two things within the poem.
Here's an example:
Whitecaps on the bay:
A broken signboard banging
In the April wind.
If you go back like 30 or 40 pages, haiku was all the rage, and all the kids were doing it.