What is a de-facto citizen, and how can I prove I'm not one?

I've recently returned from a 5 month stay within the United States. I realize i'm only allowed to be there for 6 months legally, and also that when I have returned, theres no time limit i have to wait to re-enter the United States. I plan to go for another 5 or 6 month stay, but i've read that if the US Customs and Border Patrol see that i've been in the United States longer than i've been in Canada, they'll ask me to prove that i'm not a de-facto citizen. What is a de-facto citizen, and how can I prove that i'm not one?