What is the bottom ash? What's the reason of produce bottom ash?

What is the bottom ash? 

What's the reason of  produce bottom ash?

   The bottom ash refers to the misty toner was appeared on the blank area of the print.

The reason:
 
    Charge roller dirty or wear; toner quality problems; Photosensitive drum fatigue; Environment was low temperature and drying; Print quality is not good.