Matthew 1:21

21 Mary's baby will be a boy. He will save his people from their sins. Because of that, you must give him the name “Jesus”.’

1:19Joseph thought that Mary had had sex with another man. When two Jewish people had promised to marry each other, it was not easy to stop the marriage. Joseph could have told people in a public place that Mary had done something wrong. This would stop the marriage. The law said that they should then kill Mary. But Joseph wanted to be kind to Mary. Then the angel told him that Mary had not done anything wrong, so he should marry her.
1:21Jesus saves people from God's punishment for the wrong things that they have done.