AFC stands for Automatic Filter Cleaning system.

A frequent cleaning cycle will automatically run during operation to ensure that suction performance is always ait its best. If suction performance decreases or in very dusty applications, cover the suction hose with your hand (use glows) and hold it for about 15 seconds. If suction power is still reduced take out the filter and clean it and make sure the filter is undamaged or replace the filter. 

To activate AFC, simply press AFC button ( Shown in the image). When the AFC is activated the button lights up. 

For certain applications, such as wet pick-up, it is recommended to turn off the Auto Filter Cleaning system.