What an affidavit is, and when it applies
An affidavit is a written statement sworn under oath before an authorized official. In the super visa context, it is what makes a translation acceptable when the person who did the translating is not certified.
IRCC requires “an affidavit (sworn statement) from the translator” in that situation, and explains its purpose: “This statement confirms the translator’s language skills and the accuracy of the translation.”
It has to be sworn in front of someone
It is not something the applicant can simply sign at home — “The translator must swear an oath for the affidavit in the presence of an authorized person,” which in Canada is usually a notary public, a commissioner of oaths, or a commissioner for taking affidavits.
If your translator is certified, no affidavit is needed, because the translator’s seal and membership number serve the same verification purpose.