Uart1 will provide some debugging information and is another booting option.
When the A13 powers up many images will send something out of the Uart which is a good check that anything is working especially if you dont have VGA enabled, no LCD and no wifi.
The baud rate is 115200, 8 bit and 1 stop bit.
There are several options to connect a computer to Uart1.
1. Buy the USB Serial Cable F from olimex.
2. Make your own serial cable.
To make your own serial cable you will need a DB9 connector on your computer, many computers now do not have this. If you dont have a DB9 either buy the USB serial cable from olimex or buy a USB to serial adapter.
As the voltages coming out of the A13 board are 0 to 3v3 most computers will not reliably recognize these voltages and will probably register strange characters that do not make any sense.
To shift these voltage levels to the computer levels you can use a max232 chip. The datasheet of the specific max232 chip you get will explain which pins to connect.
On the DB9 the receive (data into computer) is pin 2, transmit (data out of computer) is pin 3 and ground is pin 5. Connect these signal after they have been level shifted by a max232 chip to pin header uart1. Each pin has a description on the back.
Ensure you connect the receive from the computer (pin 2) to the transmit from the A13 board. Same goes for the transmit side.