Annotations are only used by the Plugin system in Drupal 8
Annotations contain metadata about the function or class.
Annotations are written as PHP code comments above a class or function .
How Drupal understand the annotations
Block manager scans files looking for @Block annotation tag to find block instances
Annotations are read and parsed at runtime by an annotation engine. Drupal 8 uses the Doctrine annotation parser, which turns it into an object that PHP can use.
Drupal Annotation Syntax
Drupal’s Annotation syntax is taken from Doctrines (but it is not exactly the same).
Annotations comprise of nested key value pairs and support nesting.