Ais simply a pad and pad with the same number. Meaning that UART1 is the TX1 and the RX1 pads. In the image below we highlight 5 uarts. UART1, UART2, UART3, UART4, UART5.
In Betaflight on the ports tab, you will see a list of UARTs. The UART and it’s number indicate the RX and TX for that given UART.
What Can UARTS Do?
UARTS can be used for receivers(xm+, crossfire, F.Port, SmartPort, etc…), smart audio, camera controls, GPS devices, and many others. On some boards, especially F4 boards, there are special UARTS that have inversion built into them for receivers that required inverted signals.