The Rapture is the removal of all true believers before the judgment of God. The Bible speaks about a great tribulation of 7 years that will come over this world. But before that will happen the true believers will be removed from the world in a moment of time.
1 Th 4:16–17 For the Lord himself will come down from heaven with a commanding shout, with the voice of the archangel, and with the trumpet call of God. First, the believers who have died will rise from their graves. 17 Then, together with them, we who are still alive and remain on the earth will be caught up in the clouds to meet the Lord in the air. Then we will be with the Lord forever.
Two things will happen at the Rapture:
1. All believers that died in the past will be resurrected.
2. All believers that are alive will be lifted up into heaven.
If you want to know more: Listen to this sermon.