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)