A USART is more commonly known a serial port. It’s used to transmit information between Maple and another device (like your computer, another Maple, etc.).
To use a serial port to communicate with an external serial device, connect the TX pin to your device’s RX pin, the RX to your device’s TX pin, and your Maple board’s ground to your device’s ground.
Don’t connect these pins directly to an RS232 serial port; they operate at +/- 12V and can damage your board.
Each LeafLabs board has at least 3 separate USART devices. In the most simple use case, the RX and TX pins are used to send data at a predetermined speed (or baud rate). Their usage is documented in the Serial Ports language reference page.
Which pins correspond to the USART TX and RX pins are given on your board’s silkscreen, and also in the board-specific USART pin maps available here:
If you use a particular serial port, you cannot also use its communication pins for other purposes at the same time.
We have successfully used the USART ports with an FT232R-based USB-serial converter at up to 115200 baud. Higher speeds should certainly be possible.