If you haven't customized your Board & Card sync, you may find that there's a list in your Trello board--or several! called <something> - Inbox.
When you haven't customized your sync's list mapping, there may be lists in one board that we simply have no idea where to put in another board. When that's happened, we create an inbox named <boardname> - Inbox. Rather than put the cards somewhere random, we create this list, put a card with our logo on it in the top so you don't wonder where it came from, and then put all of the cars whose destination we're unsure of from <boardname> into its specific list.