An antidote is a substance or a thing that can make people feel better who took a certain poison. The antidote will make some of the effects of the poison go away.
Antidotes are specific to certain poisons. They often are poisons themselves. As an example, Atropine, a substance that is found in the deadly nightshade is a very poisonous substance. However, by the way it reacts with the body, it is used as an antidote against some insecticides, as well as certain kinds of nerve gas.