BT Domain Shield is a product that enables you to mask your Whois information in order to add an additional layer of protection from fraudulent activity like spamming and phishing.
If your domain name has the shield installed, BT populate your Whois information with random contact details. Attempts to access the Whois data are then filtered, so that only legitimate emails are passed on to you.
BT Domain Shield is currently available for gTLD domain names (names that end with .com, .net etc). You can be purchase them when you register your domain name.