A predefined Global Snippet is a snippet that has been installed by the plugin when you first activate it. More or less that are example- (or demo-) snippets you can work with.
The plugin will ask you after the installation if it should install all the predefined snippets. If you missed this, you can always go to the settings page of the plugin and hit the “Go for it” button.
You can learn more about how to re-install the predefined/example snippets here.
Attention: The “predefined snippets” functionality has been removed in version 2.22 in favor of the new Setup Wizard.