Can some one please explain the code.

In your code "this" is used overload the constructor.The this() syntax instructs the compiler to insert the specified constructor that matches the parameter list specified.

eg:

  public class User{
       string firstName,lastName;
       int Age;

        public User(string FName,string LName):this(FName,LName,Age=0)
         {
             firstName=Fname;
             lastName=LName;
             this.Age=Age;
         }
   }
class Program
{
    static void Main()
    {
	
	User obj1= new User("abc","xyz");
	User obj2= new User ("abc","xyz",19);
    }
}

Output:
        1] abc, xyz, 0
        2] abc, xyz, 19