It is a quaint term of endearment that evokes the old charm of Singapore. Uncle and Aunty are the popular terms of greetings for and older woman or man whom one may meet during daily discourse. Your taxi driver may be 'uncle' and the shop lady may be 'aunty'. It is a local manner of speech that confounds some visitors as they wonder how Singaporean families may be so large, friendly and diverse.
(I was in a taxi recently in Singapore and the driver didn't know my destination , so I called the hotel so some one could speak to him - I passed the mobile to the driver and heard he person on the other end of the phone say 'yes uncle'.)