From a program standpoint it is an error generated by a library function when the program being run attempts to, in some way, modify the terminal enviroment but discovers that it's output is simply being piped, either directly to your terminal from a script, or back to you through WHM/Cpanel.
Suffice it to say, as hst pointed out, it is nothing to be concerned about. You will see this warning on a normal basis in any cpanel system.
If anyone is actually interested in how to get rid of this message, comment out (or remove) the line that says "mesg y" from /etc/bashrc
this line simply allows others to send messages to your shell using the write command...not exactly the most important line in bashrc....
Most people ignore this error, but recently I discovered WHM Autopilot did not handle that message well during some operations and needed to find out where the error was being generated and how to get rid of it.
(My particular problem was occuring during account creation with slave/master dns...the error would show up in the account creation report. To solve this I commented the above line out of /etc/bashrc on the master dns server.)
Thank you nuclear. The other's opinions on not being important are fine though aswell, but when I have resellers asking me what it means and that I have to constantly rexplain to them what it is it may get annoying.