Plugins can have their own bootstrap. If you have a plugin named Example, you will have a file at app/Plugin/Example/Config/bootstrap.php This file is required for all plugins and will be loaded (when active) every time the application bootstraps.
This is useful for storing basic configuration info like:
Configure::write('PluginName.my_setting_key', 'value here');
Then it can be accessed anywhere in the application by:
$myVar = Configure::read('PluginName.my_setting_key');