Assuming no issues with your ticket (might want to give DL reservations a call to double-check), best gueSSSS is that you'll have some "special" security screening before you board your ATL-UIO flight tomorrow.

The last time I got that message, I was coming home LIR-ATL-XXX, and I couldn't check in online for my flight from LIR. Sure enough, I got the SSSS on my BP and had to get secondary screening at the gate before I could board. No issues or additional time at the ticket counter, but it did require the ticket agent to print my BP (kiosk gave me the same message).