Maybe that he is not a 14th amendment citizen ie. Amendment XIV, Section 1, Clause 1: All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside.
He's saying that he is a free person not subject to the jurisdiction of the US government. Because he wasn't born in the United States, and since the United States by legal definition, areas subject to jurisdiction, is that 10 square mile of land, now smaller, called the District of Columbia, and the territories and possessions of the United States including the Commonwealth of Puerto Rico.
The United States is not a country, but a collective of countries called states. He is saying he's a free and natural person born on land known as America.
Also, when ever you check on a form that asks, "Are you a citizen of the United States?" It is asking if you are a 14th Amendment citizen. You then are consenting to the jurisdiction of the United States government.
There is so much more involved in this subject than space and time would allow, but that's the jist of it. He is most likely a "sovereign" believer.
