Read Matthew 24:36-44
36 “No one knows, however, when that day and hour will come—neither the angels in heaven nor the Son;y the Father alone knows. 37The coming of the Son of Man will be like what happened in the time of Noah. 38In the days before the flood people ate and drank, men and women married, up to the very day Noah went into the boat; 39yet they did not realize what was happening until the flood came and swept them all away. That is how it will be when the Son of Man comes. 40At that time two men will be working in a field: one will be taken away, the other will be left behind. 41Two women will be at a mill grinding meal: one will be taken away, the other will be left behind.
42 “Be on your guard, then, because you do not know what day your Lord will come. 43If the owner of a house knew the time when the thief would come, you can be sure that he would stay awake and not let the thief break into his house. 44So then, you also must always be ready, because the Son of Man will come at an hour when you are not expecting him.
Many people interpret this scripture as people being snatched up to heaven to be with the Lord. If you read the passage consistently - it is the good people who are preserved - and the evil who disappear - as in the days of Noah (vs 39).
The surrounding passages are also about being ready for Jesus to arrive.