To affirm something is to give it a big "YES" or to confirm that it is true.
The verb affirm means to answer positively, but it has a more weighty meaning in legal circles. People are asked to swear an oath or affirm that they will tell the truth in a court of law. Lawyers may ask people to affirm facts, and judges may affirm rulings. In these cases, affirm means to verify or attest to the validity of something.