In five (Pancha) places (Kshetra), lying on the river Kaveri, Vishnu is worshiped as Ranganatha. Ranga is the stage, Ranganatha is the Lord of the Stage. The stage is the world, the cosmos.
Adi Ranga, the primordial Ranga, is Srirangapatna.
The others are the Divya Desham Srirangam, Tiruppernagar, Tiruindhalur, Tirukudanthai.
Read the story about Ranganatha here