Erm… what is it? Forgive me for not wanting to download the PDF to find out, but I can't for the life of me work out why there isn't an opening paragraph explai

Erm… what is it? Forgive me for not wanting to download the PDF to find out, but I can't for the life of me work out why there isn't an opening paragraph explaining what it is, and I'm too old for guessing games. This is the second link from HN I've opened today like this.