I heard that private constructor prevent object creation from outside world.

When i have a code

public class Product
{
   public string Name { get;set;}
   public double Price {get;set;}
   Product()
   {
   }

   public Product(string _name,double _price)
   {
   }
}

here still i can declare public constructor(parameter),won't it spoil the purpose of private constructor? When do we need both private and public constructor(parameter) in code?

I need detailed explanation please.