What does Mark 2:14 mean?

The tax collector is identified as "Levi son of Alphaeus." This is problematic because no roster of disciples includes a "Levi"—not even the list included in the next chapter (Mark 3:14–19). The roster does include "James the son of Alphaeus," but since the identical story given in Matthew 9:9–13 refers to a disciple named Matthew, it's widely believed that Matthew and Levi are the same, and James' father is given to distinguish him from James the brother of John and son of Zebedee.

The text doesn't mention that Levi has had any prior interactions with Jesus. Since he works near Capernaum, though, it's a good bet that he, like Andrew and Peter (John 1:35–42), at least knew of Him prior to this moment. This would explain his willingness to abandon his very lucrative post and follow Jesus.

As a Jewish tax collector, Levi has leased the right to collect taxes on behalf of the Roman ruler, most likely Herod Antipas. In the process, whatever extra he collects serves as his salary. In Capernaum, which sits on the border of Antipas and Philip's territories, tax collectors probably tax goods and their transit, including fish. It's possible that Levi knows Andrew, Peter, James, and John, and if so, it's likely the four fishermen don't think highly of him. They do respect Jesus, however, and no mention of dissent is recorded.

The Pharisees, legalistic religious leaders, are harsh on those who do not follow the extra rules they added to the Mosaic Law, but they are particularly derisive of Jewish tax-collectors. Tax collectors are not allowed to be judges or witnesses in court cases, and they are excommunicated from the synagogue. If Jesus had limited His teaching to religious environs, Matthew would not have heard Him teach, and we would not have his gospel.