A certified copy is a copy of a document that has been legally certified in Germany. It means a stamp would be attached to the copy that proves it is identical to the original document. Many authorities do not want your original documents, but a certified copy of them- as the originals must be preserved from loss or any other damage.
During the authentication process, it is verified that the copy is identical with the original. A special stamp and the signature of the person who carried out the authentication process is added to the document as an official guarantee- this stamp must be visible on all pages of the document.
Please note: Most authorities don’t accept certified copies of a certified copy. You need to have the original document as well. For instance, if you have a certified copy from Afghanistan and make another certified copy from it in Germany, the second copy won't be accepted by German authorities.