It means that if there is a security problem or minor bug fixed in the application, then Canonical makes no guarantee they will update it to protect your system, etc.
If there is a major problem the Ubuntu community will usually issue an update though.
No, I wouldn't worry at all about monitoring new releases.
Ubuntu comes with a default set of packages installed and the package manager track those packages. If you remove a package that is installed by default, it becomes marked as "deselected". This means it was installed previously, but has been removed. In fact any package that you install and then remove becomes marked as "deselected".
This is useful for example to replicate the same packages you have on another machine or after a clean install. You can generate a list of "selected" and "deselected" packages on your system using the package manager and then load this list on another machine, which will make the package manager on it to install all "selected" and uninstall the ones marked as "deselected".
To replicate your packages selection on another machine (or restore it if re-installing), you can run this:
dpkg --get-selections > ~/my-packages
Then move the file "my-packages" to the other machine, and there run:
sudo dpkg --set-selections < my-packages && sudo apt-get dselect-upgrade
When you run the command above, all packages that were marked as "selected" will be installed in a batch and all packages marked as "deselected" will be removed, if present. This is a very handy feature.
Best Answer
If you install a package, all packages that that package depends upon are also installed. For example if you install the package vlc, it will automatically install vlc-nox. The automatically installed packages (in this case, vlc-nox) are set as "automatically installed" -- if you remove vlc, the package manager will suggest to remove vlc-nox as well (aptitude will do this automatically, if you use apt-get you can remove all automatically installed packages with
apt-get autoremove).Now, if you do
apt-get install vlc-noxyou will get the message that vlc-nox is now set to "manually installed", i.e. the package manager now thinks that you want that package specifically and not just installed it because vlc needed it. If you remove vlc, vlc-nox will therefore not be automatically removed.This does not affect updates in any way.