Java access modifiers control code visibility and enforce encapsulation

Java access modifiers control visibility of code elements, enforcing encapsulation with four levels: public, protected, private, and default. Private restricts access to the declaring class, default to the package, protected to the package and subclasses, and public is universally accessible. These modifiers are crucial for designing robust and maintainable Java applications, guiding developers on appropriate scope for classes, methods, and variables.