"Plotter" is pretty easy to decipher--it's someone who plots out their stories before they begin to write. Details, such as the characters' names, eye color, backstories, motivations, and their exact roles in the story are carefully noted. Timelines are mapped out to the exact minute (especially in murder mysteries) and charts are drawn up to show exactly what direction the story will take, including the "unexpected" twists and turns.
"Pantser" is short for "flying by the seat of your pants"--someone who just sits down, with only an idea for a story in their mind, and starts writing. They might know who the characters are, what they look like, and why they do what they do, but the details will be ironed out as the story gets down on paper. They know what the story is about and what's going to happen, but it might take a 38-hour day in the first draft to tell it (that's what editing is for, right?)
My style of writing falls somewhere in between. Of course, I have to know who my characters are and what's going to happen in the story, but plotting a novel down to its every detail would drain me of energy and creativity. I'm not a recklessly impulsive person in real life--I do like to think ahead--but when it comes to having fun, I like to give myself a little freedom to "wing it". And writing is fun!
My husband and I are planning a long weekend getaway in a few weeks. I have often said that, for me, it's not a vacation if I have to look at my watch. Having an itinerary for every moment of a vacation seems to suck the joy and excitement out of the experience. If a vacation is meant to give me a chance to relax and recharge, then being on a schedule won't help. However, there are times when it's important to think ahead to what one might want to do on vacation. Certain things--a train ride, a whale watching tour, a special dinner event--do require planning because they are being arranged by someone who isn't on one's particular schedule. It's possible to take a chance and go on vacation and just hope everything falls into place--tickets won't be sold out, reservations aren't required, etc. If you don't really have your heart set on doing this particular activity, that won't be a problem. But if you do, then some planning is sure to be involved.
Just like we have planned out a grape-picking party at an estate vineyard, a lunch time train ride, and a VIP tasting at a winery during our long weekend, a writer has to plan key events, elements, and situations in a book. The fun, unplanned things can connect those points and eventually, they will all make sense and flow into one another to make a story work (with a lot of editing to take care of those 38-hour days!)
In a way, an author is like a cruise director. We have a lot of planned activities and interesting people to meet, but not all of them necessarily contribute to the story. So take some "planned spontaneity" and enjoy the unexpected. It makes for a better story and vacation!