The informal, everyday items of clothing you wear are your street clothes. After a long, sweaty game of volleyball, most of the team members take a shower and change into their street clothes.
There are many outfits that don't qualify as street clothes, including a tuxedo or gown, a bathing suit, or a school uniform. Things you wear for exercise, sports, work, or a formal occasion are not considered street clothes. The jeans and sweatshirt or casual sundress you throw on? Those are street clothes, which get their name from the idea that they're appropriate garb for walking down the street.