This Rule can be interpreted (from the Job Fairy perspective) as not telling your colleagues anything about your personal life. Simple, yet effective. What they don't know, they can't use against you.
When I was a young help desk puke years ago, I worked with a serious troublemaker that I'll call Bob. I was trusting and not nearly as cynical as I should have been. Bob was insecure and jealous of anyone whose skill levels approached his own. Bob, under the guise of polite conversation, would ask about my family, my home life - things like that. And he would share things about his (so I wouldn't get suspicious). I thought nothing of it, until Bob started to take the information that he'd been given (over breaks, lunches, things like that), and to use it to stir up trouble against me. For example, I used to manage a commercial property downtown. He cited that as proof that I was poverty-stricken and couldn't be trusted, or that I wasn't paying enough attention to my job. I finally went to my supervisor, who I'll call Mike. Mike was ex-military, and a no-nonsense sort who looked me straight in the eye and told me it was my fault. I yelped in surprise. Wasn't it Bob who was causing problems? No, Mike said - you gave him the ammunition. It was a good lesson; I learned it swiftly. I soon went on to bigger and better things, and didn't commit the same error again.
So beware of colleagues who, with the seemingly best of intentions, want to have lunch with you and find out "all about you". Their motives might not be as benign as you think...