The verb "err" means to do something wrong; to make a mistake is "to err". "To err is human" because all people ("humans") make mistakes. "To err is human, to forgive divine" says we should try hard to forgive others because all people are human and make mistakes. Example: "I am still angry about what my manager did yesterday!" Answer: "It is best to just let it go; to err is human, to forgive divine." The world of people is "human" and the world of God is "divine". God's special power to forgive people for their mistakes is called "divine" mercy. When we forgive other people we are acting "divine". "To err is human, to forgive divine" says that we are all human and we all make mistakes so we should all try hard to forgive other people when they make mistakes. Example: "I will never forgive my mother for what she has done!" Answer: "Don't be angry at her. To err is human, to forgive divine." (Because we use the word "is" in the first phrase "to err is human" we do not use "is" again in the second phrase: "to forgive divine".) We are all people who make mistakes so to forgive others when they make mistakes is the right thing to do; "to err is human, to forgive divine."