That's your 4 finger connector.
Seriously do you have a problem?
Gm uses somewhat generic wiring harnesses rather than build a custom one for every specific option available.
It could very well be for some option your truck doesn't have. Look carefully into the end of it to see it the metal contacts show any sign of ever having been plugged into something.
The most common example is all harnesses include a clutch pedal safety switch connector, but if the truck has an automatic is simply never connected. Same for fog light plug behind the headlight switch.