Signal handlers are the places where you do the actual work of acting upon signals that are received. As a programmer, a signal handler is where you will place the code that should be executed when a particular signal is emitted by the widget.
Signal handlers are of two kinds: default handlers and user-defined handlers. The default handlers are called automatically on receipt of their corresponding signals. User-defined handlers on the other hand have to be explicitly connected to signals by the programmer (that's you!).