Playbutton is a button, the sort you pin to your clothes, which contains music and has a headphones socket and a couple of buttons to control the playback. There's an article in English about them at http://www.bbc.co.uk/news/business-11948391

There seem to be quite a few (for a quirky format) released in Japan and we have some in the database already, I've tagged 13 on https://musicbrainz.org/tag/format-playbutton/release (which would already put it higher than 10 existing formats on https://musicbrainz.org/statistics/formats ) and I can easily find more - there are 33 currently listed on http://playbutton-japan.com/collection/ for example.