[shock]

A responsibility is an obligation to perform a task or know information (authoritatively). If an object performs two tasks, it has two responibilities, and so on.

For example, an Entity part of the persistence layer in the app, has the responsibility to persist the state of the Entity to the database, but the responsibility to know the information is with an object that is part of the business logic layer. The information that is stored in the Entity to be able to persist it to the DB is just a cache of the information in the Business Object, the Entity is not responsible for it, it just holds a cache. If the same object would be responsible for both holding the information and persisting it, it would have 2 responsibilities.

This might sound somewhat confusing and useless, but it isn't so. Imagine a future where computers have a form of RAM that is not volatile. There is no need for a database, in the classical sense – whatever is in RAM when the computer is powered off/rebooted will still be there when the program resumes running.