As John introduces the feeding of the multitude, he calls the lake 'the sea of Galilee, which is the sea of Tiberias.' The double name helps readers identify the same body of water through regional and contemporary terminology.
The newer name reflects the political geography of first-century Galilee.
Herod Antipas founded the city of Tiberias on the western shore of the lake and named it in honor of the emperor Tiberius. Over time the city's prominence contributed to the lake being called the Sea of Tiberias, a name John uses again later in the Gospel.
The lake itself is the freshwater body also known in biblical tradition as the Sea of Galilee and Lake of Gennesaret.
The name quietly places John's narrative within the Roman-era landscape. An old Galilean lake now also carries the name of a city honoring Caesar.
For readers encountering several biblical names for the same body of water, John's wording also prevents unnecessary confusion: Galilee and Tiberias designate the same lake.