UART means the serial communication, aka COM port. In Next computer, it is routed to the ESP8266 WiFi module socket. But it is still possible to use it for serial communication between two Nexts, or connect other peripheral, that uses serial port in 3.3V levels, or use cheap USB <--> UART cable to send data between PC and Next.
But there are some limits:
- Data format is fixed to 8N1: 8 data bits, no parity bit, one stop bit.
- No handshake signals, such as RTS/CTS, DTR/DSR.
- Not 5V tolerant. Putting more than 3.6V will cause the Spartan6 death