Robert Browning in his famous dramatic monologue, "My Last Duchess", uses this line. Here 'smiles' suggest the smile of Duchess who was very much tender, gentle, amicable and obliging to everybody. But the Duke is very much jealous and mean minded who everytime doubted his wife. He did not like the Duchess's behaviour to everyone. She did not differ common people from her husband and she thanks everyone even for their very triffle gifts. The Duke could not tolerate this and ordered to kill his former wife and this she stopped smiling altogether.

             This line finely lights up on the character of the Duke as well as of the Duchess. We see that the Duke is very much rude, proud and jealous. He thinks that his wife is honoured by marrying him as she gets 'a nine hundred years' old name'. But Duchess is very kind from the core of heart and responsive. That's why,  she did not distinguish her husband from a common man. But for her kindness, she got the punishment of execution from her husband.