Press Kit
It's something musicians and their management send to journalists, and that journalists throw away.
More specifically, it's a package of stuff that's supposed to give journalists background information so they can write about the artist(s) being touted, as well as to convince them that there's some reason they might want to write about him/them. Once you have one, you can use it for other purposes too, like impressing relatives and people you want something from.
As to what people do or should put in it (or not put in it, never, in a million years, you idiot!), other posters who have more experience as "promotable" artists, their management or journalists can probably give you better information than I.