A single status affidavit — also called a certificate of no marriage, a certificate of freedom to marry, or soltería — is a sworn statement that you are not currently married.
Most countries require one before allowing a foreign national to marry there. The United States does not issue such a certificate centrally, so it is created as a notarized affidavit and then apostilled.
Wording requirements vary by country and some consulates insist on their own template. Tell us the destination and we will draft it to match, notarize it, and apostille it — usually in one visit.