If it is determined by the proper tribunal that a valid marriage never existed, what exactly is an annulment “annulling”?
An annulment is more accurately termed a “declaration of matrimonial nullity” and does not actually nullify anything. Rather, the annulment process determines whether what may appear to be a valid marriage is actually a valid marriage or not. If it is determined that a valid marriage does not exist then it is declared that the marriage is null - that a valid marriage never existed.
In his book Annulments and the Catholic Church Edward Peters explains that an annulment is “a juridic determination that, at the time of the wedding, one or both parties to the marriage lacked sufficient capacity for marriage, and/or that one or both parties failed to give adequately their consent to marriage as the Church understands and proclaims it, and/or, in weddings involving at least one Catholic, that the parties violated the Church’s requirements of canonical form in getting married”.