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Tagging interface

in tutorial it is said that tagging interface is designed to

Adds a data type to a class: This situation is where the term tagging comes from. A class that implements a tagging interface does not need to define any methods (since the interface does not have any), but the class becomes an interface type through polymorphism.

but the class becomes an interface type through polymorphism. whad does this line mean? can u explain me with example?