Why are birthmarks a thing?

What are we marked by? Are we not supposed to be born with moles? Is that it? And why is mine red? And do I remember right that I have (or had) two? No, it was that they switched it on me or something. I just remember growing up thinking my birthmark was this brown dot (mole) on the side of my toe, and then sometime when I was maybe even college age, seeing my birth certificate or some form from baby me time saying my birthmark was actually this reddish scar-looking thing beneath my shoulder blade and thinking, Wings!