Self-certification is when you sign a form yourself to say something is true, instead of getting an official person to say it for you. In Britain the everyday case is being off sick: for the first few days you can 'self-certify' — you fill in a short form for your employer saying you were ill, and you do not need a doctor's note. After about a week you do need the doctor. That is why the word turns up in offices, schools and HR emails there. The word is used more widely too, in the same shape: a builder can self-certify that work meets the rules, a company can self-certify that a product is safe. In every case the meaning is the same — the person doing the thing signs for it, and the system trusts them, with checks only if something goes wrong. It is a formal, administrative word; nobody uses it at dinner. The related verb is 'self-certify' and the adjective form is 'self-certified'. It is mainly British; American workplaces usually talk about 'calling in sick' or a 'doctor's note' instead.