A trigger catches a certain event, e.g. a OnOpen document event or a variable change event.
You can then define several actions that QV automatically executes when the trigger fires.
You can define these triggers and actions in Document properties - Triggers, in sheet properties and also on some chart object properties like button or text boxes.
What is it that you are wanting to achieve with a Trigger? swuehl 's description of how to set them up is spot on. Personally I would warn against them though - as making selection changes or navigation changes based on an event can confuse users - as things happen that they are not in control of.
They can be useful though, perhaps using a selection in a data island to cause something to be selected in the rest of the data model.