Hi,
I always find it easy, when I am not sure exactly what a function does, to test it with printing output to the console.
For example here you could use this
void ofApp::mouseDragged(int x, int y, int button){
cout << "test " << x << " , " << y << endl;
}
To see that when you click and hold the mouse the function will constantly fire (like the update function), but only when the button is pressed. The x and y values will refer to the position of the mouse in the viewport.