Ready-made things are standardized and bought right off the shelf, rather than being specially made. You might have to hem your ready-made curtains, but they're cheaper than having custom curtains sewn.
Ready-made clothes are the ones you buy in stores or online that come in standard sizes. Custom, tailor-made, or bespoke clothing is the exact opposite — they're constructed to perfectly fit your body. Ready-made food is store-bought, and made commercially, not homemade. You can also use this adjective to describe something that's so quick and automatic it's become a cliche, like a politician's thoughtless, ready-made response to a reporter's question.