Here Jesus ends the teaching he started in chapter 5. Judging the behavior of others is a way people can appear righteous even when they aren’t. Jesus says the path to God’s kingdom is a narrow one. It isn’t by trying to impress everyone with how righteous we are; it’s through the forgiveness that is found in Jesus. This way is hard to find because it is the only way.
We naturally pursue appearing righteous, judging others, and trying to have a relationship with God in our own way. Jesus teaches against all of that. He wants us to be righteous on the inside, accept and forgive others, and come to him in his way.