A Division I fencing tournament is the highest level of competition mandated by the United States Fencing Association.  Usually, Division I events (such as at a North American Circuit event) are open only to those fencers who have obtained a classification of "D", "C", "B", or "A" in the weapon of that event.

The Division I events at the National Championships are theoretically open to anyone, but the path to get there is rigorous--only the top 25% of the field at the divisional qualifers for Division I advance to the sectional qualifiers, and only the top 25% of that field get to compete at Nationals.  So, while a ringer might make it through one round of eliminations, it isn't bloody likely that a fencer who doesn't deserve to be there will make it through both.  The Division II and Division III qualifying paths are only half as hard--only the divisional qualifiers are held.

Winning a national Division I competition is a big deal, and they're almost always won by one of the ten or so best fencers in the country in that event and category.