Dolby Digital 5.1 refers to a sound codec which is designed to be played on five speaker channels and one LFE channel. The five speaker channels are as following: centre channel, left channel, right channel, rear left channel, rear right channel. Dolby Digital 5.1 can also be configuraed

• Single centre stand alone. 1
  
• Two-speaker stereo - single Left and Right. 2
  
• Three speaker stereo - same as above but with an additional centre speaker. 2
• Two speaker stereo but with a single surround sound speaker. 3
  
• Same as above but with three speakers.3
  
• Four speakers, but with two surround sound speakers. 4
  
• Finally five channel, with right/left/centre, plus right/left surround sound. 5
  

Now you may all be wondering what does the "0.1" refer to?. This is an additional LFE channel. LFE stands for "Low-Frequency Effects" which is usually a subwoofer. The vast majority of home cinema systems come packaged with a subwoofer.