this is because of some leftover from DOS which stand for output device.
con stands for 'console'
aux stands for 'auxiliary'
prn stands for 'printer'
These are specified in IO.SYS and date back from the early MS Dos days. Here is a brief list:
CLOCK$ - System clock
CON - Console; combination of keyboard and screen to handle input and output
AUX or COM1 - First serial communicationport
COMn - Second, Third, ... communicationport
LPT1 or PRN - First parallel port
NUL - Dummy port, or the "null device" which we all know under Linux as /dev/null.
CONFIG$ - Unknown