An addiction is an abnormally strong craving for something, which can sometimes grow so strong you have trouble living without it.
People will sometimes use the word addiction casually, like, "I have a major addiction to chocolate! Yummy!" But the word's true meaning is more serious, and refers to something that isn't so yummy, like an alcohol or drug addiction. When someone has an addiction to smoking, for example, they might not be able to stop smoking even if it causes them to get sick.