A Trigger Group is an organizational tool, like a folder, which contains the Links settings and a trigger tree you create. You cannot trigger motion clips in the Animation Trigger window until you create and select a Trigger Group.
A Trigger Group also lets you link a character, a device, and motion clips. You can trigger motion clips only when these three elements are selected or present in the Trigger Group. Once you have done this, you can create and activate a trigger tree.