Retired Documentation: You are using the documentation for version 1.7.3 which was retired in 2013. Go here for the latest version documentation or check here for your available upgrades to the latest version.
Cookie Settings
This section of the Control Panel allows you to define cookie settings for your website.
Cookie Domain
This preference allows you to set your cookie domain. The vast majority of people will want to set this to:
.example.com
Obviously you'll use your actual domain name instead of ".example.com". Note the period at the beginning as it is very important.
Setting the cookie domain like this will allow the cookies to work regardless of whether people specify your URL with the www part or only the http:// part.
If you want to restrict your cookies to a single subdomain then you may set that here.
Cookie Path
This is an optional setting. You will only need to set this if you require a specific server path for your cookies. If you run multiple installations, or have your installation in a lower folder you can specify a folder from which to make the cooke available. If you set the path to /joe/, the cookie will only be available in the "joe" folder and any subdirectories of it. It will not be available in directories above /joe/. The vast majrity of people will leave this setting blank.
Cookie Prefix
If you will be running multiple installations of ExpressionEngine on the same server then you will want to specify a unique cookie prefix for each installation. This cookie prefix will prevent the cookies from interfering with each other.