When you select a tag from a behavior's properties, that behavior now affects all objects that share the same tag. For example, if you select a tag on an event behavior like “Started Touching”, this behavior will be run whenever you touch any object which has that tag applied.
If you select a tag on an action behavior like Apply Force, it will apply the force to all of tagged objects at the same time.
Advanced Behaviors and Tags
If you have an event behavior connected to an action behavior where both the event and action have the same tag, then the event will pass the specific object that was triggered down to the action. For example, if I have a “Started Touching” connected to an “Apply Force”, and they are both associated with the same tag, then the specific object that is touched is the only object that will have the force applied to it.