For example:
<aura:handler event="c:appEvent" action="{!c.handleApplicationEvent}"/>
The event attribute specifies the event being handled. The format is namespace:eventName.
The action attribute of <aura:handler> sets the client-side controller action to handle the event.
In this example, when the event is fired, the handleApplicationEvent client-side controller action is called.
The framework allows you to handle the event in different phases. These phases give you flexibility for how to best process the event for your application.
Application event handlers are associated with the default phase. To add a handler for the capture or bubble phases instead, use the phase attribute.
In the client-side controller action for an <aura:handler> tag, use evt.getSource() to find out which component fired the event, where evt is a reference to the event. To retrieve the source element, use evt.getSource().getElement().