An affirmation is when someone swears to tell the truth before giving evidence to the court.
Anyone who gives evidence can choose between an oath (which is made on a religious book) or an affirmation (non-religious). When a child gives evidence in the youth court or in an adult court they promise to tell the truth (instead of swearing).
It is presumed that witnesses of 14 or over are to be sworn, unless there is evidence suggesting that they do not have sufficient appreciation for the solemnity of the occasion and of the particular responsibility to tell the truth which is involved in taking the oath.