SPN clarifications

We are getting below message: > ‘SQL Server cannot authenticate using Kerberos because the Service > Principal Name (SPN) is missing, misplaced, or duplicated.’ I found an article on [how to get it fixed](https://blogs.msdn.microsoft.com/sqlupdates/2014/12/05/sql-server-kerberos-and-spn-quick-reference/) . But I still have the following questions: 1. Currently our auth scheme is NTLM. How is NTLM different from Kerberos? 2. What exactly is this SPN? I am worried about my PRODUCTION servers. 3. Which is more secure and recommended by Microsoft: "NTLM" or "Kerberos"? 4. In what cases is it used? 5. If I don't fix this error, what issues I am going to face ?