Can anyone tell me what this port is for?

That connector is the IR sensor connector. On the front there is both a White LED and this IR sensor.

Review Step 13 in this guide Mac mini Mid 2011 Logic Board Replacement there should be a cable hanging in that corner of the shell with the Black, Red & White wires

If you snapped it off the logic board you might be able to re-solder it on as long at the pads are still present and not torn off. Even if they are, someone with good skills can patch things together by adding jumper wires to replace the pads.