Channel bonding in wireless LAN is the bonding of adjacent 20 MHz channels to create a wider bandwidth channel (e.g 40MHz/80MHz etc). The higher bandwidth channel in effect will have higher bandwidth and hence higher data rate supported. One such example is shown below
Fig Courtesy: Wikipedia
As can be seen from the tabular column above – for an 802.11n system – doubling the bandwidth provided doubles the data rate supported.
The 802.11n standard provided channel bonding of 40MHz whereas 802.11ac supported both 80 MHZ and 160 MHZ channel widths. 802.11ac also supported a split 80+80 MHz mode of operation.
An example image of the channel bonding scheme is provided below
One of the channels in the bonded channel bandwidth is referred to as the primary channel and the other channels are secondary channels.