The GlobalContentManager is a value which can be used to load content using a “global” content manager. Not only is this content manager available globally, but it is never unloaded. Therefore, any content loaded using GlobalContentManager will not be unloaded until the game exits.
Using a GlobalContentManager is useful for content which appears frequently in games (such as fonts) or for content which may take a long time to load/process, and should be kept in RAM to improve load times despite occupying space permanently.
For information on global content in Glue, see the GlobalContentFiles page and the UseGlobalContent page.