NOC is a No Objection Certificate from your current employer stating they have no objection to your joining a government post.
Apply for it through your office’s HR or administration, mentioning the post and the interview or DV date.
Some departments issue it only after you clear the written stages, so ask about the process early.
A few notifications allow you to submit the NOC at joining instead of the interview — check the advertisement.
Without a required NOC, some bodies may not let you proceed to document verification.