The nick is the name the user will use while they are on the server. If you use the nick=guest parameter then the users will get a random nickname. This is nice for bringing the user directly onto the server and requiring no input from the user. You can force the user to change their nickname to something else before speaking by using the 'lurk' command.
Examples:
In a web page:
<param name="nick" value="guest">
In a param file:
nick=guest