DAF stands for Detailed Application Form, which UPSC asks mains-qualified candidates to fill online after the mains result.
It collects your personal details, educational qualifications, service preferences, home state, and optional subject choices.
The details you enter in the DAF are used for the personality test (interview) and final service allocation.
Fill it carefully — mistakes in the DAF can be hard to correct and may affect your interview round.
Keep scanned copies of all certificates ready before you start, and submit before the deadline shown on the UPSC website.