After Jesus was crucified (nailed to a cross made of wood), he died and was buried in a tomb. On the third day, he was resurrected, meaning that God had restored Jesus to life.
In the New Testament of the Bible, there is a book called 1 Corinthians, which was written by a disciple named Paul. The 15th chapter of this book explains the significance of the resurrection of Jesus. It says that Jesus died for our sins (see 1 Corinthians 15:3). It also says that our faith would be meaningless if there had been no resurrection (see 1 Corinthians 15:17). And it proclaims that Jesus was resurrected (see 1 Corinthians 15:20).
Paul is one of the people who encountered Jesus after the resurrection. Paul traveled more than 10,000 miles by land and by sea, telling people about Jesus, his resurrection, and his promise of eternal life in the presence of God for those who have faith in Jesus.
Read more in the book Jesus the Miracle Worker: The 50 miracles of Jesus explained in chronological order (The Jesus Series) -- now available at amazon.