Google Hacking refers to the practice of using search engines, like Google and Bing, in order to discover vulnerable web pages and critical information. It's based on the idea that search engines index a lot of public pages and files, making their discovery a simple matter of building the correct query.
For example, it's trivial to look for a specific type of file (filetype:), on a specific domain (site:), with a specific name (inurl:), containing a certain string (intext:).
The Google Hacking Database (GHDB) was started by Johnny Long, who also published books on the matter, but is now mantained and updated at Exploit Database. I suggest taking a look at some real examples to better understand GH.
As an ending note, the origin of the term google dork, as stated by Johnny himself:
We call them 'googledorks': Inept or foolish people as revealed by Google.