This Japanese expression relates to books or short stories written for mobile phones. There is a good article published in Le Monde the other day. Nothing really new in the sense that these have been available for a while in Japan and recently introduced in Europe.
However, I was really amazed by the high level of downloads. Apparently, one the latest books (just made available in physical bookstores), Koizora, has already been read by 25,000,000 Japanese.
The buzz around Amazon's Kindle should be put in perspective.