Subject matter jurisdiction means that a court (either state courts, or Article II/Article III federal courts) has specific jurisdiction (authority) over a matter because of the specific issue at hand. That jurisdiction/authority is derived from a law, statute, or regulation that specifically grants the court that authority.
(For example, Bankruptcy Court hear bankruptcy cases, Admiralty Courts here maritime cases, Federal Courts for Federal Laws/Statutes, etc…)
In regards to Sovereign Citizen matters, just ignore what they say because they are spewing word salad they have memorized without having any context or knowledge of the actual terms they are espousing.
[Edit – clarification]
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