Wedding traditions are fun but can often seem a little strange in modern times. I love to make bridal garters for my Etsy shop, The Sparkling Cocktail, so I recently did some research as to why a bride's garter is tossed. 
While the tradition of the garter toss can still make some guests uncomfortable it is very tame compared to it's origins. Back in the 14th Century Europe it was considered good luck to have a piece of the bride's clothing. After the nuptials were made guests would tear at the bride's dress to have a piece of that luck. In some regions it was customary for the guest to escort the newlyweds to their bedroom to make sure the marriage was consummated. As proof they would leave with bits of the bride's dress, veil, stockings, and garters. Over the years this led to the groom just tossing out the garter to the rowdy guests so the newlyweds could have their privacy. Now it is tossed at the reception well before consummation to the bachelor guests and signifies the next to marry.
Often brides will wear a blue garter. The color blue has signified love, fidelity and modesty since Roman times. Flapper photo courtesy of Art Deco Blog.
