Reafforestation is the work of planting trees again on land that used to be forest, after the trees were cut, burned, or lost. It means the same as reforestation; reafforestation is the older, more British-looking form, and you will mainly meet it in formal writing, older books, and some government papers. Though actually, people who work with trees draw a line many everyday speakers skip: afforestation is planting a forest where there was not one before (a field, a hillside), while reafforestation is putting a forest back. In ordinary talk, many people just say “planting trees” or “reforestation” and do not split those two.