After looking at the crowds and having compassion on them, Jesus tells his disciples “the harvest is plentiful but the laborers are few.” He then encourages his disciples to pray to the Lord of the harvest to send laborers into the harvest. In what follows this saying Jesus prepares his own disciples to be the laborers in the harvest.
But Jesus describes the harvest as plentiful, not “ripe” or ready for harvest. There are so many people living in Galilee that Jesus cannot personally visit all of the villages to teach with authority and heal. This is the initial motivation for appointing twelve of his followers as disciples. They are going to go to the villages of Galilee and announce the coming of the Kingdom of God (Matthew 10).
A harvest as a metaphor in the Second Temple Period for eschatological judgment (threshing wheat and gathering grapes for the winepress). The “Lord of the Harvest” refers to God, but likely also Jesus as the Son of Man who will render justice at the final judgment. Even in the Parable of the Weeds (Matt 13:24-30, 36-43) the harvest refers to a future time when the wheat and the weeds will be separated, the wheat will be taken into the barn and the weeds burned on a fire (where there is weeping and gnashing of teeth). In the book of Revelation, the “harvest of the earth” refers to the devastation of God’s enemies at the final judgment at the end of the tribulation (Rev 14:14-20).
Does Jesus mean, the field is ready for harvest because the judgment is about to happen? Or does he mean there are many people in Galilee who are ready to hear the message of the messiah? The harvest is not yet ripe, as if the end of the age has already come and the wheat and weeds are about to be separated. But in some respects the harvest has begun when Jesus sends his workers out into the fields, his disciples to the villages of Galilee.
If the people are harassed and helpless, then they are exactly the people who would be looking for the Messiah to come and render judgment on their own leaders and any gentiles who are harassing them.
Who are these workers for the harvest? In the immediate context, the twelve disciples are sent out to the villages of Galilee in the very next paragraph.
There is nothing wrong with the application of this passage to evangelism or world missions. Christians ought to pray to God for him to send workers into the fields ripe for harvest. But in Matthew, that “mission field” is specifically the villages in the region of Galilee. This becomes more clear in Matthew 13, in the parable of the sower. The sower who sows his seed is Jesus announcing the nearness of the Kingdom of God; the various soils will represent reactions to Jesus’s preaching and healing in Galilee up to that point in Matthew.