What is Character.isLetter() in JAVA?

class JAVA {
    public static void main( String args[] ) {
        //simple letter
        System.out.println("Character.isLetter('E'):");
        System.out.println(Character.isLetter('E'));
        //other character
        System.out.println("Character.isLetter('*'):");
        System.out.println(Character.isLetter('*'));
        //digit character
        System.out.println("Character.isLetter('5'):");
        System.out.println(Character.isLetter('5'));
    }
}