Package removal?

[Jim Byrnes]

Shortly after installing Unbuntu I used the Synaptic Package Manager to 
install Thunderbird 2.0.0.23.  I've since manually installed version 3 
and want to remove the earlier version.

I went to the package manager and marked it for removal.  At the final 
step I see this message:

Summary:

13 packages will be held back and not upgraded

1 package will be removed

What is the significance of the 13 packages being "held back"?

Will this mess with my 3.0 installation at all?

Thanks,  Jim