D.M. means "Digital Media" so this port on a Sony television, home cinema or hi-fi system allows you to plug-in an adaptor and stream music wirelessly from another device.
The D.M. port adaptor works with many Bluetooth-equipped devices, including laptops, computers, mobile telephones, smart phones and PDAs (personal digital assistants). To work properly, the device must support A2DP (Advanced Audio Distribution Profile)/Bluetooth profile 2.0. Read the technical specification in your device manual to check that it's compatible.
The adaptor is a Sony accessory rather than standard equipment, which means you'll have to buy and install it separately. It's easy to set up as it plugs in to the D.M. port on your Sony equipment and is powered by this connection. You can find the D.M. port by checking the diagram in your Sony manual.
Sony also sells a D.M. port iPod cradle, which allows you to plug-in an Apple iPod and play music through your home cinema or hi-fi system. You can buy D.M. port adaptors from Sony dealers and the Sony website.