Affirmation in law

Affirmation is like when you promise to do something, but instead of saying it with your hand on a special book, you just say it out loud. In the law world, sometimes people need to promise that they are telling the truth, but they don't want to use a special book or swear on something that they don't believe in. So they can choose to do an affirmation instead. It's like a fancy promise that is just as important as if it was sworn on a book.