What is it? It is a milk pad though a coffee strainer thingy, (I don't drink coffee.) would probably do just as well. I have this problem when I water my lettuce and such in the pots under the grow light. They wash to the side leaving large bare spots. Or they wash to the surface and then dry out and die.
Can't remember what inspired the thought, no surprise since my memory is a atrocious.
I water over the pads and it slows the water keeping it from washing things out and spreads the disbursement. It also keeps the surface of the dirt moist longer. I don't use plastic over my pots. It is a eco thing of mine. Plastic takes too long to break down in the landfills so I try and avoid the highly disposable kind.
For the larger seeds it is no problem to do without the pads but for spinach, leafy lettuces, and herbs this works like a charm. I have quite a few new pots started for fall harvest in the sunroom. When the seeds sprout they push up the pad and I remove it. When I am through using the pads, I will wash them and put them aside for the next round of seeds. This is a must repeat idea.