GTK+ By Example/Glib/Signals and Callbacks
What Are Signals and Callbacks?
See Theory of Signals and Callbacks Tutorial.
Real Life Analogue
Think of a fortress with gates, the gates remain open unless someone is attacking. How does the fortress know of an attack, It has Signal Towers that burn a fire, and a person watching the towers, to sound a warning bell and the guards close the gate.
We need some code that works and follows the real life analogue.