This is coming from the MOTD (Message Of The Day). The MOTD is pieced together from the commands in
/etc/update-motd.d. The specific message comes from running
I have seen this happen on systems where after running
sudo apt-get update and packages are kept back
The following packages have been kept back:
linux-generic linux-headers-generic linux-image-generic
0 upgraded, 0 newly installed, 0 to remove and 3 not upgraded.
The message can mean several things e.g. there may be dependency issues etc. there is more on this in the APT HowTo.
I have managed to stop the messages with
sudo aptitude update
sudo aptitude safe-upgrade
If that doesn't work then the link above does give advice on how to track down the problem packages.