What is a "mixed child"? Why don't they "belong"?

From the text:

“Jem,” I asked, “what’s a mixed child?”

“Half white, half colored. You’ve seen ‘em, Scout. You know that red-kinkyheaded one that delivers for the drugstore. He’s half white. They’re real sad.”
“Sad, how come?”
“They don’t belong anywhere. Colored folks won’t have ‘em because they’re half white; white folks won’t have ’em cause they’re colored, so they’re just in betweens, don’t belong anywhere.