It refers to the judgement of the lawsuit against a person and its assets and not anything else like absolute property or things. Its a latin term meaning "Against a person". When in a lawsuit the judge passes a judgement against a particular person, it is not mandatory for the person to be present there though court summons him/her to be there. The judgement is enforceable wherever the person is. This is known as in personam action. It differs from "in rem" where the judgement is against the property and the court has the jurisdiction at the place of the property only.