Are you from the US? Maybe this is not popular in other countries, I don't know. Anyway it is part of a somewhat traditional wedding. Flower Girls have very expensive dresses. Basically:
A Flower Girl is usually a young child, the recommended age between 4 and 8 years of age. Her male equivalent is the Ring Bearer or Page Boy. Often the ringbearer and the flower girl are made to look like a couple, as with the best man and maid of honor. You can see a bunch of dresses from FlowerGirlDresses.com it is a Flower Girl Dress company apparently.