Refurbished products are devices (think iPhones, MacBooks, iPads etc) that are pre-owned, as in bought by somebody, or pre-used as display items. For one reason or another, the device was sent back either to the manufacturer (Apple) or to the retailer it was bought from.
After conducting any necessary repairs or replacements, the refurbished product is put on the market again. Refurbished products are sold by the Apple Store or by a professional refurbisher. On RefurbMe, you can choose to filter devices by seller if you have a favorite retailer in mind.