Can you help me figure out what "traditional mother of the groom style" means?

My husband’s son is getting married next Friday evening. I am not super close to him and have only met his fiance a handful of times. The boy is calling it formal, not super formal, but formal. The groom and groomsmen are wearing suits and not tuxedos, but it seems as though tuxedos are pretty 20th century and not the norm at even formal weddings these days. So now I am left to find a formal outfit that is “traditional mother of the groom style” and I am not sure start. All I know for sure is that I would like something that I will be able to wear again. Any ideas on where to start or styles to consider?