Hi Rohit
SPDIF means it is an Optical Out. It is also known as a TosLink connection. If your AV receiver has an Optical In jack, you can get an optical cable and use this to connect the HD STB to your AV receiver. The receiver will automatically decode the 6 channel sound and give the output through your speakers and subwoofer.
Alternatively, since the Airtel HD STB also has HDMI out, and if your AV receiver can pass through audio using HDMI, you can simply connect the HDMI out from the Airtel HD STB to your AVR's HDMI in, and connect the HDMI out from your AVR to your TV (if your TV has HDMI). The AVR will then pass through the video (or upscale depending on the model of the AVR and its features) to your TV, and give the output to your speakers.
If on the other hand your AVR does not support audio thru HDMI, then you can connect the HDMI out from your STB directly to your TV and the optical out from the STB to your AVR. Most AVRs have at least one optical in port.
Hope that helps and I haven't confused you!
---Venkat