The AT91 USB to Serial Converter Windows driver refers to a software driver that enables communication between a Windows computer and a device that uses the AT91 USB-to-Serial converter chip, typically from Atmel (now part of Microchip Technology). The AT91 series of chips are part of Atmel's line of microcontrollers, and they are often used in embedded systems and hardware designs that require USB-to-serial communication, such as interfacing with serial devices like RS232 or RS485 devices through a USB port.