Guys, can you explain what kind of adapter is this?

[welkore]

Trying to find out what the heck is this for.

 

[USAFRet]

That's a motherboard speaker.
Plug it into the appropriate connector, and it beeps on startup.
Hopefully, it only makes success beeps and not error beeps...

 

[Paperdoc]

By the way, "appropriate connector" for that is the set of pins on the mobo Front Panel header that are designated for the Speaker that your case MIGHT have anyway.