A mongolian spot is a dark area typically on darker-skinned babies... Hispanic, African-American, Pacific Islanders, etc.
The first time I ever saw one, I wigged out, wondering how the heck a baby got so bruised inside the uterus. If I'd have seen the baby next day (without seeing him/her at birth), I would have thought s/he had been beaten the night before. The dark spot looks eerily like a big bruise.
Sometimes the mongolian spot is round and right at the base of the spine (as it was with my Hispanic children), but as you will see with this baby (half-African-American and half-Asian), the marks are more mottled and diffused.
The baby's anus is at the left edge of the picture, respectfully off the side. The white you see is vernix and someone pointed out the spots are so dark, you can see them through the vernix.
Some mongolian spots fade with time; many stay for a lifetime.