What is waste pad and why it is so important?
It is a special sponge to absorb ink, located at the bottom of the printer.
What is it for?
When you do head cleaning, the printer flush the ink to unclock print-head then after that the ink flows to waste pad.
Why should I care?
The sponge has a limit to absorb the ink. Once it could not absorb ink anymore then the ink start to leak from the bottom of the printer.
How do I know that the sponge could not absorb ink anymore?
The printer counts how much ink flow to waste pad and stores this number into printer’s memory. This number named protection counter. Please refer to related post to get more detail.
What should I do if waste pad full?
The sponge is not easy to get full if you are using original/genuine cartridge. Poor quality of ink caused print-head clogged and force you to perform head cleaning.
If your waste pad full, you should send the printer to service printer to be replaced (if still under waranty). Alternatively, you can open the printer and clean the sponge.
Source: Refilling Cartridge