Alcohol tolerance is a special case of drug tolerance: with certain substances, such as alcohol, the body gets used to them. People consume substances such as alcohol, because these substances have certain effects. As the body gets used to the substance, the quantity needed to get the desired effect wil increase.
With certain drugs, the same is true for stopping: directly stopping the drug is dangerous, because of withdrawal symptoms. So the amount of the drug taken has to be reduced in steps, before stopping altogether. There are drugs that can make stopping easier, because they reduce the severity of the withdrawal symptoms.