To use and especially to maintain the server, some tools are required. To maintain a remote computer, the most important is probably the SSH tool. SSH (Secure Shell) is a nework protocol which can be used for a encrypted connection to another computer. When talking about SSH, it is also considered as the tool to connect to a computer for remote administration.
When using a Plugcomputer, there is usually no keyboard support and no display output. Thus, the whole administration runs remotely over the network, but what happens when you mess up the network configuration. This is why there is a JTAG/UART interface available which emulates the terminal of the plugcomputer. It acts as the display of the plugcomputer and can even show the output displayed during booting of the plugcomputer which gives additional debugging possibilities.