An official certifeod copy is a confirmation (with date, stamp/seal and signature) of the authenticity of the photocopy of a certificate from an authorized body.
Video: https://handbookgermany.de/en/certifiedcopies (You can select another language in the top right corner.)
The following institutions can issue an official certification in Germany: Local government offices, administrative districts, and lower administrative authorities, e.g. mayors and heads of local authorities, city councils (city halls), district administrations, courts and notaries.
Abroad, the following offices can issue official certification: institutions that issued the certificate, e.g. schools, universities, language schools, the Ministry of Education, German embassies and consulates, the cultural department of the embassy of the country in which the certificate originates, and authorities and notaries who are authorized to issue official certifications.