An affidavit is a piece of paper accompanying the translation saying something along these lines: "I certify that the translation of the documents attached is a true and accurate translation of the original."
This affidavit needs to be in English (since the translator works with Dutch <> English, he/she should have no problem writing this in English). Some translators even put this sentence at the bottom of the translation.
This is a sample of an affidavit (the one I had for my translations).
P.S. I couldn't find a translator in Germany who knew what this was. I googled a translator in Canada and contacted them. Sent him scans of my documents, he sent the original translation to me by mail (I paid him the mailing fee). But all together, it cost me way less than what I would have paid here in Germany. Paid him with PayPal, 80 CAD for 3 documents, here I would have paid 70 euros for only 1 document. And since he is based in Canada and registered with a Canadian Association of translators, there is no reason CIC will doubt the authenticity of the translation.