The two terms are Nanowrimo lingo. Silvermint in the process of discussing Game of Thrones, explains them well on Twitter:
It has to do with the behind-the-scenes process of plotters vs. pantsers. If you’re not familiar with the distinction, plotters create a fairly detailed outline before they commit a single word to the page.
Pantsers discover the story as they write it, often treating the first draft like one big elaborate outline. Neither approach is ‘right’ - it’s just a way to characterize the writing process. But the two approaches do tend to have different advantages.
Because they have the whole story in mind, it’s usually easier for plotters to deliver tighter stories and stick the landing when it comes to endings, but their characters can sometimes feel stiff, like they’re just plot devices.
Pantsers have an easier time writing realistic characters, because they generate the plot by asking themselves what this fully-realized person would do or think next in the dramatic situation the writer has dropped them in.
But because pantsers are making it up as they go along (hence the name: they’re flying by the seat of their pants), they’re prone to meandering plots and can struggle to bring everything together in a satisfying conclusion.
That’s why a lot of writers plot their stories but pants their characters, and use the second draft to reconcile conflicts between the two.
Those are among the first seven tweets in a thread of thirty.
