SPN means "Service Principal Name". In Active Directory you can set an SPN with the command line tool SETSPN. Find below some usage examples which I use often:

• setspn -Q http/www.mywebsite.com ==> Check if the SPN "http/www.mywebsite.com" exists
• setspn -L MyServer ==> Show all SPNs for MyServer
• setspn -X ==> Search for duplicate SPNs in the current domain
• setspn -S http/www.mywebsite.com MyServer ==> Set the SPN for a specific Server object in AD. This also checks for duplicate SPNs

SPNs can also be viewed and edited in AD Attribute Editor under the attribute

"servicePrincipalName"

of an AD object.