The term 'Plugins' is referring to any types in Umbraco that are found in assemblies that are used to extend and/or enhance the Umbraco application. Plugins can also be added directly registered to their specific 'Resolver' if the plugin type is not public or if the Resolver type doesn't support finding types in assemblies.
What is a Resolver and what kinds of Resolvers are there?
Creating a single object and multiple object Resolver.
Using the PluginManager to lookup types in assemblies to register in Resolvers.