What does Mark 9:46 mean?

Like verse 44, this statement is not found in this place in the oldest and most reliable transcripts. When it is included, it and verse 44 are identical to Mark 9:48. Jesus quotes from Isaiah 66:24 which describes the fate of sinners facing God's final judgment.

Jesus is saying that we should take practical steps to prevent real sin. The examples He gives—to cut off hands or feet, or pluck out eyes—are all things we do to ourselves. Our first reaction when facing temptation should be to control ourselves, not other people. That's not always our first instinct. In contrast, when faced with a fellow believer who can afford a nice car, we may sing the praises of a more modest life to cover up our own jealousy. When beset by sexual temptation, we may rail against people who dress in a way that grabs our attention. When driving, we may get angry at other drivers, pedestrians, and the traffic engineers who set the timing on traffic lights rather than admit we have an issue loving other drivers made in the image of God.

Whatever temptation we are faced with, we need to go to God with it first. It may be that He will lead us to gently rebuke someone who unwittingly tempts others to sin; Jesus does say that it would be better to die than lead someone into sin (Mark 9:42). But we must never blame them for our lack of self-control. Our sin nature is our own, and is to be dealt with through the fruit of the Spirit (Galatians 5:22–23), practical steps, the encouragement and prayers of fellow Christians, and the work of the Holy Spirit.