Actually, I take it back, I was using a smoothing plane (No. 4) not a jack plane.
A No. 4 smoothing plane is basically a perfectly flat surface 9.5" long and 2.5" wide, with a perfectly straight blade that sticks an adjustable distance out of a slot in the middle of the flat surface. You set the blade so that it's almost not sticking out of the slot at all, so that when you push it along a piece of wood, it shaves off a piece about as thick as a piece of paper. Then what happens is this: if there are any low spots in the piece of wood you're planing, the plane is perfectly flat and the blade (in the middle of the plane) can't reach down into the dip because it is held level with the rest of the surface by the plane's sole (what the bottom surface is called). If there's a rise in the surface, the plane just cuts the rise and not around it because around it the rise holds the sole off the surface of the wood. Basically, a plane cuts very even, thin sheets off of a surface, removing any rises in the wood, until you've cut the wood down to a perfectly flat surface.
That may not be the best explanation ever, but hopefully you'll get the idea.