Charcoal is made by burning wood on a fire that is starved of oxygen. The process drives almost all of the moisture out of the wood, leaving a fuel that is easy to light and burns at a higher temperature than wood does.
Charcoal burning used to be common on the Forest. The wood was stacked in a mound that was covered in turf or clay. A small hole in the covering at the top and at the bottom of the mound was made to allow just enough air to feed the fire without burning the wood completely. Making charcoal was a dirty job, but it was also a skilled one—if you let in too little air you wouldn’t convert the wood into charcoal, and if you let in too much air the wood burned away completely.