This is a List<GameCondition> which has some helpful methods for managing the data at runtime. You can have multiple Lists of this type on an object, to hold various lists of GameCondition data, such as:
public GameConditionList conditions = new GameConditionList();
public GameConditionList awards = new GameConditionListItemObjectList();
// Perhpas "Awards" are permanent conditions that have periodic effects, but are
// handled differently in other aspects of your game, like the UI, than the main
// conditions list.
Both GameModulesActor and GameModulesInventoryActor have a conditions list.
If you inherit from GameModulesActor or GameModulesInventoryActor, you can use this list and everything will be handled automatically.