Those are the people "left behind". After all the Christians are safe up in Heaven, Jesus comes down to Earth with His Holy Army of legions of Angels. They will murder all of the non-Christians, and the lake of blood will be miles wide and "as deep as a horse's bridle". (Revelation describes the Apocalypse in great detail).

Then the murdered non-Christians will burn in Hell for all time (with all of the other non-Christians from the history of the Earth, many billions of people). The vast majority of humanity to ever exist will experience eternal torture.

Christians can watch their torture from Heaven, and listen to their torment, too, and talk to them as they are tormented. They can even smell them... God loves the smell of burning flesh like incense (that's why there were animal sacrifices before Jesus).

Over the centuries, Christians have focused on just a few words from Jesus, especially when proselyting and for "public relations". The vast majority of people have no idea of the awful truth in the Bible. Many Christians don't even read it. I have 12 years of Bible studies from Catholic, Assembly of God, and Baptist schooling. Everything I have said has accompanying scripture.