In the context of civil law, possession is a person's control or ownership of a thing. Common examples of things that are possessed are personal property such as cars, jewelry, televisions, and land.
Possession is important for criminal law because it can determine whether or not a person can be held accountable for a crime. For example, if a person is caught in possession of a certain drug, then legal ramifications can follow.
Possession basically just means that a person has control over something. For example, a man who owns a motorcycle is said to be in possession of a motorcycle. A woman who owns a diamond necklace is said to be in possession of a diamond necklace. Crimes can occur when one person tries to steal the possessions of another. Crimes can also occur when a person is in possession of an illegal substance such as certain firearms.