A bonafide certificate is a written proof issued by your school, college or employer stating that you were genuinely a student or employee of that institution.
In government job applications it is sometimes asked during document verification as extra proof of education or service.
It is not a substitute for your degree or marksheets — it only supports them.
You can get one by applying to your institution’s office with your enrolment details and ID proof.
Ask for it well before document verification, since institutions can take a few days to issue it.