TLDR: When a variable, method, or class member is declared `private` in programming languages like Java, CPP, or C Sharp, its access is restricted solely to the containing class. This encapsulation ensures that the internal state or behavior of a class remains hidden and is only modifiable through controlled mechanisms, enhancing security and maintainability.

https://en.wikipedia.org/wiki/Access_control

In Java, declaring a member `private` means it cannot be accessed or modified directly by other classes or even subclasses. For instance, a `private` field can only be accessed using getter or setter methods defined within the same class, allowing developers to enforce rules or validation. This approach aligns with the principles of object-oriented programming by safeguarding the internal state and reducing unintended side effects.

https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html

While `private` improves encapsulation, excessive use can hinder inheritance and reuse, as it restricts access even to subclasses. Properly designed APIs balance encapsulation with flexibility, often using `protected` or package-private access where collaboration between related classes is necessary. Declaring members as `private` is a key practice in creating robust and secure software architectures.

https://docs.oracle.com/javase/specs/jls/se20/html/jls-6.html