Jesus explains his mission in the first part of Matthew.
1. He came to preach
2. He came to call sinners
3. He came to bring salvation

The Law and the Prophets - shorthand for the Old Testament. He did not come to abolish it, it matters. He warns people who try to set it aside. Jesus is saying that the OT is a book about him, and he fulfils its prophecies. Ignoring what Jesus has done, though, means that we have missed his purpose.
1. Not less, but more
2. Not outward, but inward - God is concerned with our motives and Have we offended someone? We have to go by God's standards, not our own. We need to deal with our thought life. Many Christians have problems with pornography.
3. Not us, but Jesus. We sometimes fail badly here, but there is hope. It is Jesus who fulfils the OT law. He died to bring us to God.