What does John 4:38 mean?

The prior verses are meant to be both comforting and challenging to Christians. In the physical field, it is common for one person to plant the seeds, while another eventually harvests the finished goods. Harvest is a time of joy, and so the work of the harvester tends to get more glory. And yet, both are required for the crops to be brought in.

Jesus has made a comparison between fields ready for harvest and the approaching crowd of Samaritans (John 4:35). These men are approaching thanks to the testimony of a Samaritan woman (John 4:29–30). There's good reason to think that these people have heard something of the gospel from others (John 4:25). In other words, this is a field which has been seeded and only needs to be harvested. The disciples didn't teach the Samaritans about the Messiah, and they certainly didn't bring anyone back to meet Jesus.

This is why Jesus reminds them that their task—"reaping" for the sake of heaven—is not something they themselves worked for. Others have done the work to this point, but now the responsibility is on the disciples to finish.