A primary sense is the core, basic, literal meaning of a lexeme.
A primary sense is generally the first meaning that comes to mind for most people when a lexeme is uttered alone. Usually it refers to an actual physical thing, an action, or a characteristic of a referent.
The word jungle has a primary sense meaning "land covered with dense growth of trees, tall vegetation, and vines, typically in tropical regions, and inhabited by predatory animals".
This is the meaning of jungle that generally first comes to a person's mind. It refers to an actual physical thing.