Each channel in the 2.4 GHz frequency is usually 20 Mhz wide, which is the channel width. What it means is, a device cannot project a radio signal at EXACTLY 2.412 GHz (well theoretically it could, but then you can't squeeze much data in it with such a narrow spectrum).
So in practice, Channel 1 (2.412) actually occupies frequencies from 2.402 all the way to 2.422 - 20MHz Channel
The reason for the channel width is to host more data 'sub-carriers' meaning the 20Mhz width frequency is further divided into smaller partitions (52 sub-carriers), each sending its own data in parallel. This in turn maximizes data throughput. (Note that previous WiFi technologies used Frequency Hopping, so the channel width is the channel 'within' the hops could occur, google up FHSS if you're interested)
Imagine taking the whole visible light spectrum and dividing them into 7 colours of the rainbow, each colour transmitting a different stream of bits, each colour will he considered a subcarrier, and the entire visible light spectrum the entire channel width.
