A more thorough explanation:

Definition: To confirm or ratify.

Examples:

• An appellate court can affirm the ruling that was the subject of the appeal. In doing so, the court agrees that the prior ruling was valid and must stand as rendered below.
• Courts, administrative boards, and other similar bodies have used "affirm" to mean "approve."
• Government officials, witnesses in civil or criminal trials, and others who are required to take an oath may choose to state that they "affirm" the content of the oath rather than "swear" to it. This is true, for example, of the oath of office of U.S. Senators.

These examples illustrate how "affirm" is used in a legal context to confirm or approve something. In the first example, an appellate court confirms the ruling of a lower court. In the second example, courts and administrative boards approve of certain actions or decisions. In the third example, government officials and witnesses confirm their commitment to the content of an oath.