What does this apt message mean?

[Knute Johnson]

What does it mean that packages have been held back?

Thanks,

pi at raspberrypi ~ $ sudo apt-get upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages have been kept back:
   gnome-accessibility-themes gnome-themes-standard 
gnome-themes-standard-data
   libgail-3-0 libgcc1 libgfortran3 libgomp1 libgtk-3-0 libgtk-3-bin
   libgtk-3-common libjavascriptcoregtk-3.0-0 libpango1.0-0 libstdc++6
   libswscale2 libwebkitgtk-3.0-0 zenity zenity-common
0 upgraded, 0 newly installed, 0 to remove and 17 not upgraded.

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Knute Johnson