Anything preventive hinders or stops something, especially diseases.
When you prevent something, you stop it from happening. Likewise, anything preventive is used to ward off a bad outcome. Wearing a warm coat is preventative: it keeps you from getting cold. Saving your money is preventive because it will keep you from the poorhouse if you lose your job. Most often, this word is used in medicine. Preventive (or preventative) measures keep you from getting sick. Stretching before a workout is preventive, because it can keep you injury-free.