Abstinence (pronounced ab-stuh-nuhns) is the state of refraining from using drugs or alcohol or from engaging in any activity thought to be harmful. It is usually used in reference to people who have recovered from chemical dependencies, especially alcoholism, as in "The former alcoholic has achieved abstinence for over ten years now." The word "abstinence" also means to not engage in sex, as in "Some parents want their children to be taught abstinence in sex education classes."
The philosophy of most 12-Step groups such as Alcoholics Anonymous, Narcotics Anonymous, Cocaine Anonymous, and others who support the recovery of people dependent on chemical substances is that once you go through withdrawal and stop using them, you have to remain in a permanent state of abstinence for the rest of your life. If you ever use your drug or drink again, you will "trigger" a new round of dependency and have to go through the entire process of recovery again. A newer philosophy of recovery is that relapsing or re-using your old drug or alcohol is a normal part of the process, and you should expect to do that a few times before you finally achieve abstinence.
Some people are able to recover from addictions and drink in moderation, and there are even support groups for these kind of former alcoholics and addicts. However, the conventional wisdom is that people who are prone to addictions, either through their genetics or because they live in a drug- or alcohol-tolerant environments or both, cannot drink or use drugs even in moderation, without starting a new downward cycle into addiction. For these people, the only way to permanent recovery is permanent abstinence.