It’s the distribution of sound. Let’s say that we have two speakers, one on our left and one our right. Now, let’s play some music. If you notice closely, what comes out from the left speaker isn’t the same with what comes out from the right one. Usually, the right speaker has the complimentary/secondary tracks, while the left has the core elements of the music piece. That doesn’t mean that what comes out from the right speaker doesn’t come out from the left. We listen to the main vocal track from both speakers, but some of the additional tracks are slightly louder on the left or on the right. Same principle applies when we have more speakers. Movies that are edited and exported in 5.1 or 7.1 follow more or less the same rule of thumb with 6 or 8 speakers respectively.