As you probably have noticed already, during all of our discussions of 802.11 and Wi-Fi, the only two frequency bands that are currently in use are the 2.4 GHz and 5 GHz ranges. The IEEE 802.11 standard outlines the use of particular frequencies within the 2.4 GHz and 5 GHz ranges. Since the frequencies are a bit challenging to remember all of the time, the IEEE put these frequencies into groups, which are known as channels.
A frequency of 20 MHz is like a small water line; if we increased the size of the water line to 40 MHz, there would be an increased in the capacity of water supported by the line. In the 802.11 standard, an access point either can use a single 20 MHz frequency for transmitting data or can combine multiple 20 MHz frequencies to achieve a higher data rate.