Galilee is the hill country and lake district of northern Israel, and it holds the formative ground of two religions that were not yet separate when the events in question happened.
The gospels place most of Jesus's public activity here: Nazareth, where he grew up, Capernaum on the lake shore, and the villages around it. Galilee in that period was rural, Jewish, agriculturally productive, ruled by a client of Rome rather than directly, and within reach of Hellenised cities its inhabitants mostly did not enter. E. P. Sanders and others have argued that the setting explains a good deal about the movement's early shape: an itinerant teacher, audiences of villagers and fishermen, and a message about the kingdom of God delivered a long way from the temple.
The second beginning came after the catastrophes. The temple was destroyed in 70 CE and the Bar Kokhba revolt was crushed in 135, after which Jews were barred from Jerusalem, and the centre of Jewish life moved north. The Mishnah was compiled in Galilee around 200 CE, the Jerusalem Talmud took shape at Tiberias over the following two centuries, and the Masoretes who fixed the vowel pointing and cantillation of the Hebrew Bible worked at Tiberias later still.
Both traditions therefore have a Galilean phase, and neither is well understood without the other, since they were competing for the same inheritance in the same small country.