the disciples were together with the doors locked: The disciples were afraid that they might be punished for following Jesus.
Peace be with you!: This was a common Jewish greeting (see also 3 John 1:15). But Jesus was doing more than just greeting his disciples. He was offering the Messiah's peace and delivering the gift of his kingdom (see Isaiah 9:6; 52:7; see John 14:27; 16:33).