$rtname is the string that RT looks for in-mail messages to figure out to which ticket a new piece of mail belongs.
Your domain name is recommended, so as not to pollute the namespace. Once you start using a given tag, you should probably never change it otherwise, mail for existing tickets won't get put in the right place.
This regexp controls what subject tags RT recognizes as its own. Be very careful with it. Note that it overrides $rtname.
If you're not dealing with historical $rtname values, you'll likely never have to enable this feature:
You should set this to your organization's DNS domain, for example, fsck.com or asylum.arkham.ma.us. The linking interface uses it to guarantee that ticket URIs are unique and easy to construct.
$MinimumPasswordLength defines the minimum length for user passwords. Setting it to 0 disables this check.
$Timezone is used to convert times entered by users into GMT and back again. It should be set to standard Unix timezone.