Pile burning in the context of the forest service is the burning of piles of organic waste like branches, limbs, leaves, needles and other debris from clearing fuels from the forest. Often they will chip the larger limbs but pile up the rest of the debris. When conditions are satisfactory they will then ignite the piles and burn them while being closely monitored by fire crews.
The USFS has been burning many piles recently along Big Pines / N4. I am glad to see they are staying on top of this and using the opportunity to burn while the snow is still on the ground and the conditions are favorable.