When you spill the blood, you should cover it with earth. We learn from here that we don’t just put dirt on top of the blood after the Shechitah (ritual slaughter); we prepare the ground beforehand with earth so that the blood falls onto the dirt. Afterwards, you put more dirt on top of it. So there are two layers of dirt. The blood doesn’t fall directly onto the ground with dirt put on top of it; rather, you put two layers, one under and one above the blood.
Here we’ll talk about what happens if the blood did not fall directly onto the dirt, but onto a surface that is unfit for Kisui HaDam (covering the blood). The Mishnah says that if blood was splattered onto a wall or left on the knife, and after the Shechitah there is an obligation to cover it, he has to scrape the blood off the wall or knife so that it falls onto the ground, and then cover it with more dirt. So you should put it onto the dirt and then cover it with more dirt after that. What’s the situation in which the above halacha (Jewish law) applies? The only blood that could be covered is the blood in question. There’s no other blood besides the blood on the wall or the knife. If there’s other blood that can be covered, a person is exempt from covering the blood on the wall or the knife because you could fulfill the mitzvah (commandment) by covering that other blood. In other words, when it comes to Kisui HaDam, only some of the blood of the animal has to be covered. So if some blood fell on the ground, on the earth, and it’s enough to cover that blood, then you could disregard the blood that is on the wall or on the knife. But if there’s only blood on the wall or the knife and nothing else around, you’re obligated to use that blood for the mitzvah.