Pantsers, as opposed to plotters, write by the seat of their pants. Pantsers don't know anything about where the book is going, and may not even know much about the characters, before they start typing (or writing) away. It may seem scary, but pantsers live for that flash of insight that makes them go, "HEY! This is an awesome twist!" or, "What'll happen if I drop this man-eating lion in front of my hero?"
I used to be a total pantser. I loved that jolt of energy I got just from a sudden idea that went right from thought to paper without any editing in between. And hey, sometimes you can get a great surprise in that way - a surprise even to you, the writer! It can lead to some wild and memorable writing.
Now, I think I'm a hybrid between the two, and most writers are. It's nice to have a loose structure as a road map for my book, but sometimes I'll write a scene without an exact idea of its contents. I'll know where it needs to GO, but not how it'll get there, and I still sometimes surprise myself. Whatever way you write, I encourage you to try it the other way, or even mix them, just for a scene or so. You may be surprised at the results, and it could jog you out of a writing rut.