The first part of the number refers to the number of channels that are present in a soundtrack or the number of channels that a Home Theater Receiver can provide. These channels reproduce a full range of audio frequencies, from high frequencies to normal bass response. This number is usually 5, 6, or 7.
In addition, another channel is also present, which only reproduces the extreme low frequencies, referred to as Low Frequency Effects (LFE). This channel is designated as .1, due to the fact that only a portion of the audio frequency spectrum is reproduced. In addition, this LFE channel requires the use of specialized speaker, called a Subwoofer, that designed only to reproduce extreme low frequencies, and cuts-off all other frequencies above a certain point, usually 100HZ to 200HZ or above.
So, next time you see the terms Dolby Digital 5.1, Dolby Digital EX (6.1), DTS 5.1, DTS-ES (6.1), you will know what the terms are referring to.
NOTE: You will also run into some home theater receivers that are labeled as having 7.2, 10.2, or even 11.2 channels. All the .2 designation means is that these receivers have two subwoofer outputs. You don't have to use both, but it may come in handy if you have a very large room, or are using a subwoofer with lower power output that you desire.
For a more detailed explanation of Surround Sound and what the terms Dolby Digital, Dolby Digital EX, and DTS mean, check out my resource article: The History and Basics of Surround Sound.
In addition, for more information on Subwoofers, check out my resource article: Subwoofers - What You Need To Know