The wheat represents Christians, Those who are baptised with the HS.
the Chaff represents those who reject Christ, Will be baptized by fire (that shall never be quenched, there will be weeping and gnashing of teeth)
John is saying he baptizes in water, But his baptism does not really matter, what matters is how we are babtized by Christ, we have a choice, Holy Spirit, where he will gather the wheat into the barn (heaven) or fire, where he will burn it with the fire which will never be quenched (hell)