What does John 4:37 mean?

Depending on the timing, it's entirely possible for crops in a field to be planted by one person and harvested by another. Jesus is describing the approaching crowd of Samaritans (John 4:30) by using the imagery of a wheat field, white and ready to be harvested (John 4:35). In this specific case, the people are being brought by the Samaritan woman Jesus was just speaking to (John 4:7–26).

This same woman mentioned the Messiah in her conversation with Jesus (John 4:25). She also referred to Messiah when speaking to the townspeople (John 4:29). At the time, the ministries of Jesus and John the Baptist were not only successful, but they were also attracting attention from the local religious leaders (John 1:19; 4:1). This means that the Samaritan woman, and the other townspeople, might have been "planted" with gospel seed by others.

Now, with the chance to hear and learn more about Jesus, these men and women were ready to be spiritually "harvested" and brought into a relationship with Christ (John 4:30–42). This should be both encouraging and challenging to believers. Our efforts to plant the gospel are never wasted: others may well complete what we've begun. And we should be sensitive to those who have been "seeded" with the gospel, so we can take opportunities to lead them to Christ.