What does it mean when a couple Renew Wedding Vows?

It doesn't mean anything. Wedding vows do not expire where they need to be renewed. (Ecclesiastes 5:4, 5; Matthew 5:37) Still, some people do it to show that if they could do it all over again, they would pick the same person. However, they would be better off and save a lot of money if they just told each other that privately one evening.