Many physical games (TTRPGs and similar) will have multiple files available for download after purchase. Sometimes, the number of those files is quite big, with, say, a core book, a supplement, a set of basic character sheets, a set of expansion character sheets - and maybe 3 versions of each of those in different formats, or different layouts for different kinds of printing etc.
I honestly don't understand why this hasn't been added yet with this website existing for how long it has, but could a "download all files" function be added soon, please?
1. I strongly recommend you read Apocalypse World - it's a great game in its own right, but it will also set you up for engaging with any PbtA games in the future.
2. To actually answer your question though, hold is a sort of temporary currency, in most cases meant to be spent within the ramifications of the move that generated the hold (a move will be explicit if that's different). In the vast majority of cases, when a move tells you to "hold 3" or another number, it will then tell you how and what to spend that hold on. (do let me know if that's still unclear)