DAF stands for Detailed Application Form, which UPSC asks prelims-qualified candidates to fill before the mains exam.
It collects your personal details, education, service and cadre preferences, and optional subject.
You must fill it carefully — your service preferences in the DAF decide your allocation if selected.
There is a separate DAF for the personality test (interview) stage.
Missing the DAF deadline means you cannot appear in the mains, so watch the UPSC website after the prelims result.