This baseball term refers to the head coach of the team. This individual, more commonly known as a manager, is responsible for setting the batting order, assigning field positions, general strategy, and everything else involved in game management. One of the field manager's more important game duties is choosing the starting pitcher and then making game-time decisions regarding whether to bring in relief pitching. In addition to managing the game, this individual is also responsible for managing the team and ensuring relationships are maintained, egos are kept in check, and other needs of the players are met.
In semi-professional, minor and college leagues, managers have a greater role in the development of players. Most teams prefer to hire former players to become field managers. These players will often rise through the coaching ranks before becoming a manager, but there have also been instances of players becoming sports analysts for TV before becoming a coach or a manager. The term "field manager" comes from the days when the individual's primary duty was managing what happened on the field.