What does Mark 12:39 mean?

The "best seat" in a synagogue is in front, with one's back to the chest that holds the Torah. People who give feasts feel honored if a scribe and his followers join them. They honor the scribe in return by giving him the best seat—even above the hosts' parents. Jesus' half-brother James will insist that such behavior has no place in the church, and that the people the world honors are usually those who blaspheme Jesus and oppress His followers (James 2:1–7).

Jesus has already talked about the how inappropriate it is to presumptively take a seat of honor at a banquet (Luke 14:7–11). If we assume a prominent position, the host may find someone more important and publicly shame us as he moves us to the foot of the table. But if we take a humble seat, the host may honor us by moving us up. Not that we should seek such honor, of course; but it's better to assume a humble position than to be embarrassed when we're moved there later.

Matthew more specifically says that the scribes sit "on Moses' seat" (Matthew 23:2). There is debate as to whether this is a specific seat in the synagogue, but that's unlikely since there were no synagogues at the time of Moses—only the tabernacle. More likely, it means the role of an authoritative spiritual teacher who teaches the law and judges how it should be followed. As scribes, it's a valid job description. The way they fulfill their duties, however is dishonorable and ungodly.