A more thorough explanation:

Promissory estoppel is a legal doctrine that allows a party to recover damages based on a promise made when the party relied on that promise and the reliance was reasonable. This means that if someone makes a promise to you and you rely on that promise, they may be legally obligated to fulfill that promise even if there was no formal contract.

The Supreme Court has recognized promissory estoppel as a "state law doctrine creating legal obligations never explicitly assumed by the parties that are enforceable." This means that even if there was no formal contract, a promise made under promissory estoppel can still be legally binding.

If someone breaches an obligation created by promissory estoppel, a court can choose to assign either reliance damages or expectation damages. Reliance damages compensate the party for any losses they suffered as a result of relying on the promise, while expectation damages compensate the party for what they would have received if the promise had been fulfilled.

For example, let's say that a company promises to give an employee a raise if they complete a certain project. The employee works hard and completes the project, but the company does not give them the promised raise. The employee could potentially sue the company for breach of promissory estoppel and recover damages for the lost raise.