Pantsing (also known as winging it) is the term Wrimos use refer to writing without a fixed outline (an outline that the writer will force themselves to follow) . While creating a fixed outlining before November first is allowed and even encouraged, some Wrimos prefer to go without.
Pantsers vary greatly in extremity. The Extremist Pantser does absolutely no planning at all, they do not even come up with a vague idea before november 1. Others come up with an idea before November 1, but lack an outline of any sort. Others come up with an idea and a synopsis or outline before november 1, but the outline/synopsis is not fixed. Still others do little or no pre-november planning, but find time to write an outline during November, however, once again, the outline is not fixed.
Planners are the opposite of Pantsers.
Pantsers, especially extremist pantsers, are prone to dares, adoptables, and other NaNoWriMo tributes such as Mr. Ian Woon. Typically, it is harder for a pantser to win than it is for a planner to win, and a pantser's [week two] is often worse than a planner's week two.