It's meta-data, or data about the class. It's like a post-it note you can stick on the class. Later others can check to see if they are there and read what they say.
At one level it's a nice replacement for "marker interfaces" like Serializable. We can
test to see if a class has an annotation instead of whether it implements an interface.
At another level, annotations can hold important information that tells a development tool how to generate code, or a runtime framework how to handle an object.
