Checkout the Wikipedia explanation in the context of computer science and programming.

Basically I think that they just mean they want to have some plugins that are the standards. Like there might be 100 gallery plugins, but one will be canonical, the default plugin. They will test those canonical plugins before major releases, so that end users can be confident that they are stable and won't break when they upgrade.

In this case, it seems they are referring to more "official" plugins like Akismet, but from what has been said, I get the impression that these could just as easily be third party plugins as well.

I think this would be a good thing all around. It expands the options for end users, so that they can have confidence in those extra features, without bloat, and without taking over functionality that plugin developers have been providing.

I can see how this would also be helpful if a canonical plugin developer was to stop development, it would be even more likely to be picked up by another developer or adopted as an official plugin. Hopefully leading to smoother transitions in these cases.

This would be a good way to give plugin developers more of the limelight that theme developers tend to hog! But I can see how the other 99 gallery developers could feel left out in the cold...