A pro-drop language (from "pronoun-dropping") is a language where pronouns can be omitted when they are in some sense pragmatically inferable (the precise conditions vary from language to language, and can be quite intricate). The phenomenon of "pronoun-dropping" is also commonly referred to in linguistics as zero or null anaphora.

In everyday speech there are often instances when who or what is being referred to can be inferred from context. In a pro-drop language, the pronouns that in other languages would have those referents can be omitted, or be phonologically null. Among major languages, a clearcut case of a pro-drop language is Japanese (featuring pronoun deletion not only for subjects, but for practically all grammatical contexts). Mandarin, Slavic languages, and American Sign Language also exhibit frequent pro-drop features.

Some languages are only partially pro-drop in that they allow deletion of the subject pronoun. These null subject languages include many Romance languages such as Spanish, Italian, Occitan, Catalan, Portuguese, and Romanian (French is the most notable exception).

Source: Wikipedia

About drop non drop...NOTHING!!!