This setting is designed to protect your nickname from being used by other users. When set in the normal mode a user must match an access list entry to utilize the nickname. The user is given 60 seconds to identify to the nickname. If the user fails to identify in that time then the nickname is changed to a temporay Guest nickname and a services agent will be used to hold your nickname to prevent its being immediately used. You can release the nickname by using the RELEASE command.
When this is set to high users must identify to the nicname prior to using the nickname. As soon as services detects that the nickname is in use with an unidentified user it will change the nickname to 'Guest'. This eliminates the 60 second grace period that the normal setting provides.
Example:
/msg nickserv set protect high
/msg nickserv set protect normal
/msg nickserv set protect off
This command was formally called kill but was renamed to better describe its function and sound more user-friendly. If you are willing to let others use your nick when you are not using it, then leave protect off. You can always retrieve your nick whenever you want it, see the nickserv recover command. However, if you use your nick often, you may want to keep protect on, so that people will not be confused by having other people using your nick. If you are a network operator, you should definitely keep protect on. Note that if protect is off, people can use your nick, but services does not recognize them as you or give them your access.