This is a phrase used in some sports, including baseball, to describe the piece of a trade between two or more teams that has yet to be decided. This allows the trade to be processed by the league and for players to begin changing teams. This will typically only occur if the missing piece is a small one and both teams are willing to negotiate that piece at a later date in order to consummate the bigger parts of the trade. In some cases, the missing piece is an actual player that needs to be selected, and other times it is more general and refers to a prospect in the minor leagues.
The idea that a trade can still occur even though all the pieces have not been finalized helps teams make complex deals involving many players. In this situation, terms are set out in the trade agreement which qualify what type of player, the skill level, or even their years of experience in the minor or major leagues. It is up to the teams involved to set out those terms. Once they are set, Major League Baseball will be able to enforce the stipulations when the time comes.