Id files are plain text files containing a playful description of oneself.
Before the World Wide Web was invented, and long before social network services came into existence, people on BITNET used to send each other Id files as a way to introduce themselves. They were decorated in creative ASCII art (even if it actually was EBCDIC back then) and contained the typical personal information that a profile on a social network page would contain today.
The term Id file comes from the filetype used on the IBM mainframe operating systems which were the most common BITNET hosts at the time. Your ID file would be named after your login or nickname (as in FRED ID—the name and type were separated by a space, not a dot).
The file content often included machine-parsable data in a common IBM tagged format (colon tagname dot value, e.g. :name.Fred :location.Boston). Many users accumulated large collections of ID files for their online contacts, and interfaces to popular applications such as BITNET RELAY could access your Id files and their content during Instant Messaging (chat) sessions.