What is an I/O Module?

I was looking at some products at allelectronics.com and came across this unit...

https://www.allelectronics.com/cgi-bin/item/IDC5GFR/283/OPTO_22_I_0_MODULE_.html

What is an I/O module and what is it used for?

I/O = Input / Output.

The device in your link is a digital input module.
It takes a digital input from the harsh outside world and opto-isolates it to the nice clean and safe world on the input of a microprocessor or W.H.Y.

The outside world signal is a voltage in the range 35 to 65 volts (probably a nominal 48 volts), the isolated side is a standard 5volt for TTL compatability.

As this is the only item of this type they appear to have, I am gussing that they bought someones purchasing screw-up for a bargain price!

JimB