Firstly, it is important to understand the term 'estoppel.' Essentially, an estoppel is a concept and legal term that will prevent someone from denying or contradicting something that they had said or done in the past. It is used as a legal tool to ensure that no one is left at a disadvantage because of someone else's inconsistencies or attempt to skew the truth.
Now, a promissory estoppel is a type of estoppel that prevents someone from doing or acting in a way that goes against something that they had already promised to do, whether it was in a formal or informal manner. This type of estoppel is usually found in a contract. Say you sign a lease contract that says you must pay your rent on the first day of every month; this is a promissory estoppel that ensures you will do what you promise to do in your lease.
A promissory estoppel is only used when the hypothetical breaking of whatever promise was made would be extremely detrimental, whether financially or otherwise, to one of the parties involved.