A label is a component of a Google Analytics event, and is used to provide additional information for events that you want to track. A label is optional but is highly recommended when setting up event tracking.
Labels can add another layer or reporting for user interactions with page objects. For tracking purposes labels can have the same category and action but will can then be grouped separately based on the label given. The label name is up to you, but all labels are listed independently from their parent categories and actions and if you use a duplicate label those metrics will show up together.