Jesus says that the Father removes fruitless branches and prunes fruitful ones so that they will bear more fruit. He then tells the disciples that they are already 'clean' through his word.
The language combines agricultural practice with a deliberate Greek wordplay.
Grape cultivation requires careful pruning because unchecked vine growth can consume resources without producing useful fruit. Dead or unproductive material may be removed, while healthy growth is cut back to promote fruitfulness. The Greek verb kathairo, 'prune' or 'clean,' is related to katharos, 'clean,' in the following verse. Jesus therefore moves naturally from vineyard pruning to the disciples' cleansing through his word.
The agricultural background prevents pruning from sounding like arbitrary damage. A skilled vineyard keeper cuts with fruitfulness in view.
The wordplay then carries the image into discipleship: the Father's cultivation and Jesus' cleansing word are directed toward lives that bear more fruit.