A marijuana clone is the genetic duplicate, the identical twin so to speak, of the mother plant that it came from.
Most marijuana grown for consumption comes from the female plant. Male plants do not produce as much of the cannabinols and terpenes that give flavor and potency to marijuana. The mother plant is the healthy female marijuana plant that the clone cutting is taken from.
In order to clone a mother plant, the grower makes a careful cut to remove a stem and leaves from the mother. This is the cutting, which is then planted in a growing medium to cause the cutting to revert to the root-growing stage of plant life. In other words, you take an adult stem and turn it back into a baby plant.
The clone has the exact same genetic makeup as the mother plant, barring any occurrence of mutation. However, the growing environment can give the cloned plant different taste and potency characteristics from the mother plant. Usually these are small differences if they occur and do not affect the predominant characteristics that came from the mother plant.