What does the following apt-get response mean?

[John Conover]

What does the following apt-get response mean?

    root at mymachine:/ 520# apt-get upgrade
    Reading package lists... Done
    Building dependency tree       
    Reading state information... Done
    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.

Why won't it upgrade it?    

Thanks,

    John

-- 

John Conover, conover at rahul.net, http://www.johncon.com/