Use the adjective ready-to-wear to describe clothes that are made in standard sizes and sold in stores. If you buy a pair of jeans at the mall and put them on as soon as you get home, they're ready-to-wear.
Almost all clothing is ready-to-wear — the opposite is tailor-made clothing, which you can also call bespoke or made to measure. These much fancier and more expensive garments are sewn especially for the wearer's body, while ready-to-wear clothes are more affordable and — inevitably — fit less perfectly. In French, the term is prêt-à-porter, "off the rack" or "off the peg."