A recorded soundtrack or live reinforcement may include multiple channels of sound. A channel of sound is a single source of sound with a defined location in a sound field.
Imagine yourself in a space with a stage in front you. If a single individual or a group of people were talking from the left corner of the stage, your brain in conjunction with your ears would process the source of sound being located on the left side of the room. Now, if the group or individual move to the right corner of the stage, even with your eyes closed you would know that the source has shifted to the right side of the room. This spatial source is a single channel of sound.
If two groups of people were talking on both corners of the stage, you would hear two different sources from left and right sides. This would effectively result in two channels of sound, one being from the left and the other from right. If there were multiple spatial sources made up of instruments, vocals or natural sounds each coming from a distinct direction, each of these would create a channel of sound.