What is $(this) in document.ready ?

What is $(this) in document.ready ?

Hi, 

I just wonder what is $(this) inside the document.ready refer to? I try to parseHtml and get the object by using $(this) without success. I try to search to forum here but since it too generic keyword I get no result. 

Can someone here guide me or tell me what is $(this) refer to?

Best,
Shin