What is master amplifier?

By using a powerful 100% digital Sony® S-Master® amplifier and DSP (Digital Signal Processing) technology, realistic surround sound can be generated from just two front speakers. Unlike other "virtual surround" technologies, rear sound is generated from the front two speakers using sophisticated digital sound processing and does not need to be bounced off of side walls. No matter what the size or shape of the room or what's on the wall, it's possible to hear sounds from behind you without needing to place speakers behind you.

This is from Sonystyle.com

Basically in order to create surround sound in a two or three speaker sound bar, or to create clearer sound in a 5.1 or 7.1 setup; The sounds are digitally ordered and separated in order to both create distinct sound effects, instrumentation, or dialogue. There is no analogue conversion like in most systems, so the full complexity of the sound can be preserved.