what is enum in java

public class MyClass {
  enum Level {
    LOW,
    MEDIUM,
    HIGH
  }

  public static void main(String[] args) {
    Level myVar = Level.MEDIUM; 
    System.out.println(myVar);
  }
}
what is enum in javapublic enum AlertLevel {
	Undefined(0),
	Ok(1),
	Warning(2),
	Alarm(3);
}// A simple enum example where enum is declared 
// outside any class (Note enum keyword instead of 
// class keyword) 
enum Color 
{ 
    RED, GREEN, BLUE; 
} 
  
public class Test 
{ 
    // Driver method 
    public static void main(String[] args) 
    { 
        Color c1 = Color.RED; 
        System.out.println(c1); 
    } 
}
// Java program to demonstrate that enums can have constructor 
// and concrete methods. 
  
// An enum (Note enum keyword inplace of class keyword) 
enum Color 
{ 
    RED, GREEN, BLUE; 
  
    // enum constructor called separately for each 
    // constant 
    private Color() 
    { 
        System.out.println("Constructor called for : " + 
        this.toString()); 
    } 
  
    public void colorInfo() 
    { 
        System.out.println("Universal Color"); 
    } 
} 
  
public class Test 
{     
    // Driver method 
    public static void main(String[] args) 
    { 
        Color c1 = Color.RED; 
        System.out.println(c1); 
        c1.colorInfo(); 
    } 
}