As lolo said, an apostille is a stamp on your Mexican marriage certificate that certifies you are legally married. When you bring back the marriage certificate to Canada, it is considered a legal document that you may use to change your last name with (if you wish to do so) as well as other things, such as a driver's license, etc. Some resorts include the apostil charge in their wedding package fees, for instance, the Iberostar includes the apostil charge in their pricing. I hope this helps!