If you look at your headphone jack, you may notice there are black rings on it. Those rings separate different channels. One ring means only one channel (one channel and ground). Two rings means you have 2 channels (left, right, and ground) and 3 rings means there's an additional channel for a microphone.

Sending different signals down the different channels means each speaker plays a different sound. By changing the relative volumes (and timings by a very small amount) our brains are tricked into thinking the sound is coming from a certain point in space, including moving from one side to the other.