What is the meaning of 'promissory estoppel' as interpreted by the High Court of Australia (HCA) in Waltons Stores (Interstate) Ltd v Maher (1988) 164 CLR 387? What are the six requirements for the application of 'promissory estoppel' as set out by the HCA in this leading case?

Answer the above questions.

Your answers primarily focus on Waltons Stores (Interstate) Ltd v Maher (1988) 164 CLR 387; however, you are also required to refer to other principles from case law and statutory provisions, where relevant, to substantiate your discussion.