What does John 15:3 mean?

This comment comes in the middle of Jesus' analogy of the vine and branches (John 15:1–2). In prior verses, He mentioned that "branches" which do not produce fruit will be cut off and burned. This corresponds to those people who appear to be followers of Christ, but who are in fact just dead, lifeless impostors.

To both reassure and clarify this point, Jesus returns to a point He made after washing the feet of the disciples (John 13:10–11). Those who have saving faith in Christ are "clean," as are the eleven men to whom Jesus now speaks. These "clean" ones correspond to branches that will eventually become abundantly productive. Those which are not "clean" are impostors like Judas, without saving faith, and correspond to the branches destined to be discarded and destroyed (John 6:71; Mark 14:21).

Jesus inserts this comment to clarify the "dead branches" reference: it is not to Christians somehow losing salvation or falling into damnation. Jesus is explicitly reassuring the disciples that they are not among the dead branches. As this analogy continues, we see further explanation that bearing spiritual fruit is evidence of a life-giving connection to Christ. It is not a prerequisite for being attached to the True Vine in the first place.