Jesus went to the lake (the Sea of Galilee), which prepares for the events in 4:1–5:43. The large crowd that came to see John the Baptist (1:7) was smaller than the crowd that came to see Jesus.
Idumea was a region in southern Judea, settled by the Edomites after the exile in Babylon.
Tyre and Sidon were Phoenician cities on the Mediterranean coast that Jesus later visited (7:24, 31).