Okay kiddo, so "trash the dress" is a fancy way of saying that someone is choosing to do something fun and silly in their wedding dress after they've already gotten married.
They might jump in a pool, roll around in mud, or even just run around outside and get dirty!
It might seem a little weird to want to "trash" such a fancy dress, but it's a way for people to let loose and have fun after the stress of planning a wedding is all over. And don't worry, they usually still have plenty of pictures of themselves looking nice and clean in their dress from the actual wedding day.