What does this "Canonical does not provide updates..." mean in my package manager?

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.

It means Canonical does not official support the package, meaning it is a package built and contributed by community members.

In short Canonical has no responsibility regarding the said package. Though it may be updated if it's importance of a high priority.

For future reference:

http://www.ubuntu.com/project/about-ubuntu/components

Some minor exceptions notwithstanding, an Ubuntu release does not get updates with new upstream versions of software once it's out. The updates you get with Update Manager are either updates that fix major, high impact bugs, or security updates. The difference between the main and Universe components in this regard is that Canonical guarantees updates for software in main, wherease updates to software in Universe are provided by the community on a best effort basis.

There are a couple of different types of software you can download and install from Ubuntu Software-Center or Synaptic.

Official: This is software downloaded directly off the Ubuntu servers that is either maintained directly by Canonical or by the Ubuntu community. Both have a very strict process ensuring ensuring that the software works and gets security updates quickly.

Canonical only supports the core of Ubuntu while the vast majority of software is supported by the Ubuntu community.

3rd party: This is software you either directly download off the internet and install (e.g. by double-clicking a .deb-file) or software from a repository (software-channel) you manually added to you Ubuntu installation.

The footnote you see, only means that the package doesn't belong to the core of software maintained by Canonical.

Canonical isn't responsible for the upkeep of the packages, that's all it means.