Trigger function are functions that are run when a certain event happens. What's actually happening is the operating system waits for you to press a key or cause some other change in the devices, and then sends a signal to the process. This wakes it up, and runs the trigger function.
Trigger functions in event driven programming are mechanisms that decide what code to run when a specific event occurs, trigger functions are used to select which event handler to use for the specific event that has occurred. Many devices have trigger functions with a specific event that runs for it. For more information get help at