On typical computers such as Unix, Mac, and Windows, every program has three standard streams available at all times, and can open more streams as needed to access files or I/O devices:
Stdout and stderr can each be directed to different destinations to separate program error messages from normal output.
On embedded systems, input and output is typically much more limited than on a PC or larger computer. On the Vex there is no stderr stream. The stdin and stdout streams are connected to the same simple serial interface used to upload code to the Vex from the IFI loader or vexctl.
This serial interface is known as a USART (Universal Synchronous/Asynchronous Receiver/Transmitter. The term serial means that it transmits all the bits over a single wire, one after another. (A parallel interface, such as an old parallel printer cable or ATA disk cable, typically transmits 8, 16, or 32 bits at the same time over separate (parallel) wires.)
In order to display output from the Vex controller, your PC must be connected to the Vex serial port, and be running a terminal program to read and display the character data the Vex is sending. There is a simple terminal included in the IFI loader. However, I recommend the Bray++ terminal for Windows users, and Cutecom for Mac and other Unix users.