The term thin can have different meanings depending on its context.
In a general sense, it refers to something that is not thick or has little substance; lacking in bulk or volume. For example, "This paper is too thin to write on."
When used in relation to physical appearance, thin usually describes someone who has a low body weight or is of slender build.
In addition, the term can also be used metaphorically to describe something that is lacking in quantity or quality; for example "The food at this restaurant is so thin it's not even worth eating."