This is a utility which provide commandline interface to the serial port connected devices.
this is some what similar to the hyperterminal in windows
which also used for modem communications
set device(eg, ttyS0,) set baudrate say (9600 or parity hardware flow control etc).and etc...
save the configuration file
and in the root prompt
will connect device which you configured in the minicom.
(router interface with linux mechine is also done using minicom)