What is the settling of blood in the lowest parts of the body?

That's called "livor mortis."

When we die, our blood stops pumping through our bodies.
And when it stops, gravity causes it to settle toward the ground.

So, pretend that you die while laying on your stomach.

After you die, the blood in your body will stop pumping,
and it will gather to the front of your body,
because you are laying on your front.