In Java, access modifiers are keywords that control accessibility of classes, interfaces, variables, and methods. There are four types of access modifiers in Java:

1. Public: When a class, method, or variable is declared as public, it can be accessed from any other class.

2. Private: Private access modifier is the most restrictive. When a class, method, or variable is declared as private, it can only be accessed within the same class.

3. Protected: When a method or variable is declared as protected, it can be accessed within the same package or subclasses in different packages.

4. Default (no modifier): When no access modifier is specified, it considered as default access. This means that the class, method, or variable is accessible only within the same package.

These access modifiers help to control the visibility and accessibility of classes, methods, and variables Java programs.