A force (GUI player group) is an object which is capable of holding between 0 and any number of players.
A force is the same to a player as a group is to a unit.
Generally natives which affect more than one player but not all ask for a force.
A lot of the GUI functions which need forces actually preform a single player native for all players in the force or preform a global action desyncronized for the players in the force.
Forces are used to make handling numerous players more easy.
Be aware that although you can make a force out of a single player, you can not make a single player out of a force. This is because the force can be set to hold only one player (although it can also have more), while a player represents just 1 player.