As it has been said before, it varies from airline to airline. In my airline the Flight Deck dings the cabin to let me know we're ready to take off and they waiting for me to give them "cabin secure for take off".
Overall it's a communication thing using al the ding dongs, chimes and lights possible without having to use the a/c's PA systems.
Now the ding dong that annoys me the most is the toilet call bell... To all the pax out there: THAT IS NOT A BUTTON YOU CAN PRESS TO DRY YOUR HANDS!!!! OR FLUSH THE TOILET FOR THAT MATTER!!
Is it the little person with a tray on the button making you think otherwise??
