Physically the term "channel" refers to the medium through which data or signals are transferred. In the case of music files, it's a communication channel through which audio signals are transmitted from the source of the player toward the listener.
An audio file may contain one, two, or more channels. The value in the Channels section of the metadata of a file is the exact amount or the number of channels within the file. One Channel is typically described as mono, whereas more channels may refer to stereo.