Show screen ID

So that I can add a custom message, I want to be able to show the ID of an admin screen.

Mustering all my limited cut-and-paste abilities, I can do it with code adapted from something else. Although it works, I’d like to understand what I’m doing, and thus be able to modify it to what it should be.

function show_screen_id( $contextual_help, $screen_id) {

# Uncomment this to see actual screen
echo 'Screen ID = '.$screen_id.'';
}
add_action('contextual_help', 'show_screen_id', 10, 2);