What does this apt message mean?

[Robert Heller]

At Tue, 14 Oct 2014 17:53:54 -0700 "Ubuntu user technical support,  not for general discussions" <ubuntu-users at lists.ubuntu.com> wrote:

> 
> 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.

'apt-get upgrade' only updates packages whose version numbers don't 
change -- that is 100% totally safe and non-API breaking updates.  'apt-get 
dist-upgrade' updates packages that might have version number changes and 
*possibly* API breaking updates (I'm not totally sure).

If you do a 'sudo apt-get dist-upgrade', then the 'kept back' packages will 
get updated.

> 

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