TXT-records are used to hold descriptive text.
They are often used to hold general information about a domain name such as who is hosting it, contact person, phone numbers, etc.
One common use of TXT-records is for SPF (see http://www.open-spf.org).
To create a new TXT-record, right-click a zone in the left list in the DNS Records window, and select "New TXT-record" from the pop-up menu.
This record type is defined in RFC1035.