A clone is a cutting, such as a branch, taken from a living marijuana plant, which will then grow into a plant itself.
A typical clone is about 6 inches in length, give or take, and after cutting it off the first plant—called the mother plant—the clone is put into a medium such as a root cube, and given a hormone to encourage root growth. After roots develop, it is then transplanted into a pot or the ground, and it will grow like any weed plant.