Methods, constructors, fields, interfaces, and classes can have access modifiers. The general rule is that in case there is no modifier, the scope of the method, constructor, and so on, is the package. Any code in the same package can access it.

When the private modifier is used, the scope is restricted to the so-called compilation unit. This means the class that is in one file. What is inside one file can see and use anything declared to be private. This way, inner and nested classes can have access to each other's private variables, which may not really be a good programming style, but Java permits that.

The opposite of private is public. It extends the visibility to the whole Java program, or at least to the whole module, if the project is a Java 9 module.

There is a middle way: protected. Anything with this modifier is accessible inside the package and also in classes that extend the class (regardless of package) that the protected method, field, and so on, is in.