You will see/hear the term "channel" used by me or the book very often. Basically, it means the "wire" that's involved, whether it's a copper wire, a fiber optic made of glass, or a wireless frequency range.
Closely associated with the concept is a channel's carrying capacity, and an analogy can be drawn with water pipes (it's odd I suppose, because water and electricity don't mix, yet they exhibit a similar behavior in this sense). Basically, the bigger the water pipe, the more water it can carry per unit of time. A similar notion holds true for copper wire: the bigger the wire, the more electricity it can carry. Furthermore, it also applies to signals representing data (since they are electricity, after all). A bigger wire is a bigger "channel" with more bandwidth. It means it can carry a wider or higher range of frequencies on it and thus it has more carrying capacity for data, essentially.
Once again, I think more of this will be explained in a later chapter.