Events, such as mouse clicks or DOM mutation events can be handled for TDL widgets in a way very similar to that of XEL handlers attached to widget instances on a page. The handlers specified for TDL widget definitions will be there for every instance of that widget. Any XEL event handler that may be specified on a specific widget will be called before any handler defined in TDL. The TDL event handlers reside in the TDL namespace instead of the XEL namespace.
Event handlers allow you to respond to events that occur on a widget. Code within the handler is executed when the specified event type is triggered.
handler tag describes an event that you want to handle for the widget and what functionality to execute ...