What is the One Class, One Responsibility Principle?

Question

I would like to learn about the One Class, One Responsibility principle. I have found some articles about it, but without examples. It would help me if you can giv

Answer 1

Every thing in a class should be related to what the class should be responsible for.

If it is a class named MailSender, it should only know how to send the mail. Code that creates the mail content should not be inside it.