What are enums and why are they useful?

Question

Today I was browsing through some questions on this site and I found a mention of an enum being used in singleton pattern about purported thread safety benefits

Answer 1

What gave me the Ah-Ha moment was this realization: that Enum has a private constructor only accessible via the public enumeration:

enum RGB {
    RED("Red"), GREEN("Green"), BLUE("Blue");

    public static final String PREFIX = "color ";

    public String getRGBString() {
        return PREFIX + color;
    }

    String color;

    RGB(String color) {
        this.color = color;
    }
}

public class HelloWorld {
    public static void main(String[] args) {
        String c = RGB.RED.getRGBString();
        System.out.print("Hello " + c);
    }
}