public

In short: public makes an element visible from everywhere.

In more detail: Public classes, methods and attributes can be used by any other class. The method public static void main is the entry point of a Java program.

In Depth

public class Account {
    public double balance;        // changeable from anywhere (usually unwanted)
    public void deposit(double amount) { /* ... */ }
}

Public attributes should be avoided; better are private attributes with getters and setters. See access modifiers.

See also: Access Modifiers, private, protected