What is is.
The identity of the client.
What it says.
It tells you the identity of the client, a string "which identifies the owner of that connection on the server's system" (RFC 1413, ex Authentication Server Protocol (RFC 931))
What may cause trouble.
You probably won't ever see this field containing something other than "-" (not set). The Identification Protocol (RFC 1413) defines a way of establishing the identity of a client. Personally, I never ever saw this field being used.