RS232 is one of the standard for serial communication.
It is able to be used for long distance communication,
as compare to using parallel.
For basic concept of serial communication,
you might like to visit the following website.
There is quite some picture illustration for basic understanding.
**broken link removed**
You might also like to visit the following website,
for a detail and indepth knowledge of RS232 communication.
This is my favourite website. https://www.beyondlogic.org/