When a bride wears a garter to her wedding its a fun thing to do, some weddings have a ritual whereby the groom removes the garter from the brides leg, sometimes with his teeth!!
Apparently a very very long time ago it was thought to be good luck if you got to take a piece of the brides dress home with you and not all brides enjoyed having their dress ripped to shreds by her guests so someone thought of the garter holding up the brides stockings, the bride removed the garter and threw it to the guests and whoever caught it would have good luck. Nowadays the groom throws it to all the bachelors at the wedding and whichever one catches it is next to be married. A bit like the bride tossing her bouquet.
So its now a tradition to give a bride a garter to wear on her wedding day. You can buy the bride her garter here: https://www.myfairlady.ie/products/garter-tlg514-blue-boxed