What does this screw/bolt do?

[mfox]

So there are 2 underbody panels at the front of the car under the engine.
The one closest to the front bumper. ( not the one you remove to drain oil)
Dead centre has a indent with 2 bolts in it.
Can anyone tell me what the black bolt does?
The one that's behind the bolt with the washer on it.
Mine is loose and I can turn it by hand.
All others look like they hold they panel on.

 

[robz32]

that plastic nut basically slides into that bolt in order to keep the panel from falling when re-attaching all the underside panels after performing service.

 

[mfox]

Thank you