3G single band is the 3G 2100MHz technology that connects your device to the Optus mobile network. If you’re using a 3G single band device your phone relies on the 3G network unless you’re in a location that doesn’t have 3G coverage. When that happens you’ll automatically roam on the ‘next best’ network. Until August 1st 2017 the ‘next best’ network is a 2G network.
Unfortunately if you rely on the 2G network, as of August 1st 2017 you will no longer get this additional coverage. This may not be a problem for you – but if it is we’d suggest buying a 3G dual band or a 4G enabled device so that you don’t experience problems with your service.