what's this mean?

[baza]

baza at barbelith:~ $ sudo apt-get dist-upgrade
Reading Package Lists... Done
Building Dependency Tree... Done
Calculating Upgrade... Done
The following packages have been kept back:
  x-window-system-core
0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.
baza at barbelith:~ $



Anyone know what this is about??

Baza