A line formed by the ‘just married’ couple, their parents and their honor attendants that allow them to greet each guest in turn after the wedding ceremony.
A receiving line is optional can be formed at the ceremony exit or at the entry of the reception site.
The alternative to a receiving line is for the couple to visit each guest table during the wedding reception.
Merriam-Webster Online Dictionary
receiving line (noun) a group of people who stand in a line and individually welcome guests (as at a wedding reception)