Generally, I/O is an acronym for “Input / Output”. Likely these connections can be used for either depending on your mixer configurations. If the connections are phone plug “tip-ring-sleeve” unbalanced type, chances are these are designed as “insert” jacks to add external equipment (compressors, expanders, special effects, reverb, etc.) into the mixer’s audio signal chain. The sleeve provides the shield, the tip may provide a channel output signal to external processor equipment, and the ring of the phone plug provides the loopback input from the external gear back into the signal chain within the mixer. [Or the ring and tip functions might be reversed, with the same intent.]
You don’t mention what model or manufacturer “mixing desk” you’re using, so it’s a bit difficult to determine how to assist in any further detail.
If you have further queries regarding this issue, it would help us to know the specific equipment you are using.
Hope this helps!