As God had breathed into nostrils of the man formed from the dust of the ground, making him a living creature (Genesis 2:7), Jesus breathed on the those frightened disciples who had locked themselves in.
He said to them, "Receive the Holy Spirit." We remember he had promised to send them an Advocate (14:16, 26; 15:26; 16:7), the Spirit of truth to abide with them and in them.
He assigned to them the authority to forgive or not to forgive.
To have a church that continues to do the work of Christ, they must be at peace with each other. I am deeply troubled by the way some of our modern congregations are using this verse to expel people they find disagreeable. Yet, I can agree with the expulsion of some persons who have caused damage and would have continued to be harmful if the congregation or the denomination had not cut them out of positions of responsibility.