According to the UiPath documentation, an action is generated when a process reaches a point where human intervention is required. The action is then sent to the Action Center, where it can be viewed and managed by users. The action will have the status of Unassigned until it is assigned to a specific user, either manually or automatically1. The process will remain in a suspended state until the action is completed by the user2.
References:
Action Center - About Actions
Action Center - Managing Actions