Las = them in this sentence. Do you know those girls? = ¿Conoces esa chicas?
"Those girls" is placed in front for emphasis.
Since DL accepts "guys" for chicos, I experimented here with, "Those chicks, do you know them." DL didn't accept chicks. My question: does anyone say "chicks" any more? And is that a valid translation? Edit: DL does except guys for chicos / gals for chicas
Chicks would be slang, politically unacceptable, and demeaning to many. For these reasons, while it might be said, I don't think it would ever be an acceptable alternate answer for girls/women in a formal language lesson.
I agree with you. From strictly a lexical point of view, however, some words in English come from Spanish: buckaroo from vaquero, gringo from greenback, and I wondered if chicks might have come from chicas.