This passage of Mark describes the calling of the first disciples (literally: learners), people who would come to play an important role in stories about Jesus. These disciples constituted the core group of Jesus eschatological community people who believed his message and would be there to lead in the new order that would follow the apocalypse.
Disciples are called and play a role at the earliest stages in all of the gospels, demonstrating their fundamental importance. Jesus message was not one about a purely mystical or heavenly Kingdom of God; instead, the message had a core social dimension which required a community of dedicated believers.
Unlike traditional stories from Jewish and Hellenistic cultures, these disciples were not already out seeking a master to follow. Jesus is the one who takes the initiative to call them to his side. They are also not students of philosophy or religion; instead, they are regular men engaged in everyday activities in support of their families and communities. All of this they leave behind in favor of a personal commitment to Jesus and in order to establish a new family with him at the head.
One wonders why they chose to follow him. Perhaps if they were portrayed as having listened to him for a bit, the entire scene would make sense; yet all we have is him walking up, telling the two pairs of brothers to come with him, and they do just that. Christians have traditionally thought of their decision as reflecting an inner knowledge of Jesus true identity, but there is nothing in the text to support that and its a weak rationalization.
What was the attraction? What did they find appealing about the idea of being fishers of men indeed, what did they think that phrase meant? They must have really liked the idea, or Jesus, or both because otherwise they wouldnt have left their jobs at a moments notice like that. Its a shame that the author never explored this a curious fact, considering that Mark was supposed to have been a companion of one of the disciples.
Also curious is how Johns gospel relates that Andrew started out as one of the followers of John the Baptist. When he heard John declare Jesus to be the chosen one, though, he switched allegiance and went to get his brother Simon so they could both became followers of Jesus. This was all before the imprisonment of John the Baptist, the reverse order from what we see here in Mark.
We will keep reading about Galilee because that is the region where Jesus does most of his work. Galilee is a very small area, being only about 45 miles from north to south; according to Josephus, it had only 204 villages in its entirety. At the time Galilee was one of three administrative areas split up among the sons of Herod the Great: Galilee, Judea, and Samaria. Under the control of Herod Antipas, Galilee was also the area where modern Judaism attained its definitive form both the Mishnah and the Palestinian Talmud were created in this region.