Can anyone please tell me what this error means and how to fix it

Products.Add(new product());
Products[0].Code = "ANW135";
Products[0].Description = "Coffee Mug";
Products[0].Batch.Add(new batch());

// The above line has the following error when I run the program
// Error = System.NullReferenceException was unhandled
// Message="Object reference not set to an instance of an object."

class product
public String Code;
public String Description;
public List<batch> Batch;

class batch
public Int64 Cost;
public Int64 Rebate;
public Int64 Freight;