A more thorough explanation:

An affirmant is a person who testifies under affirmation instead of under oath. An affirmation is a solemn declaration made under penalty of perjury, but without an oath. It is a pledge equivalent to an oath but without reference to a supreme being or to "swearing."

While an oath is "sworn to," an affirmation is merely "affirmed," but either type of pledge may subject the person making it to the penalties for perjury.

During a court trial, a witness may choose to testify under affirmation instead of under oath. This means that they make a solemn declaration to tell the truth, but without invoking a religious or spiritual belief.

For example, a witness who is an atheist may choose to testify under affirmation instead of taking an oath that includes the phrase "so help me God."

Another example is when a witness who is a Quaker chooses to testify under affirmation because their religious beliefs prohibit them from taking oaths.

These examples illustrate how an affirmant is a person who chooses to make a solemn declaration without invoking a religious or spiritual belief.