"A class should have one, only one reason to change."
– Robert C. Martin
This means that one class should have only one responsibility. If a class is doing multiple things, this is a violation of the Single Responsibility Principle (SRP). For example, if we have a class named Student, it should only be responsible for student-related data. If the Student class needs to be modified when changing anything in the Teacher class, the Student class is violating the SRP.