Mystery Plug

I was under my 05 Trailblazer LS doing doing an oil change when I noticed this loose plug.

I have no idea where it goes. or what I plugs into. it has old grease all over it so it has been unplugged for awhile but I never noticed it till now. I'm hoping someone can steer me in the right direction to getting this annoying issue resolved. Once I know what this is for or some general idea I can go about cleaning and reinstalling if it severs a necessary enough function. thanks in advance for any advice and help