One of the most common meanings of jungle is land overgrown with tangled vegetation at ground level, especially in the tropics. Typically such vegetation is sufficiently dense to hinder movement by humans, forcing travellers to cut their way through.
As a metaphor, jungle often refers to situations that are unruly or lawless, where the only existing law is called "survival of the fittest". This reflects the view of "city people" that forests are such places.