This specific Bible verse appears twice in Us: little Adelaide passes a homeless man on the boardwalk holding up a cardboard sign with the verse, and later during the Tethered uprising, his doppelganger carves "11:11" into his own forehead. Generally, the verse refers to God's warning to the Jews that he'll turn his back on them if they continue to worship false idols.
Per the King James version of the Bible, it reads: "Therefore thus saith the Lord, Behold, I will bring evil upon them, which they shall not be able to escape; and though they shall cry unto me, I will not hearken unto them."
As for why Peele included it, the verse could be seen as a warning from the Tethered that they were about to "bring evil upon" the people above ground. "11:11" pops up again at other points in the film, though, like when Jason points out the hour to Adelaide on a clock in his bedroom. The numbers are also four of the same, which could be yet another emphasis on the importance of duplicates in Us.