A more thorough explanation:

Estoppel is a legal principle that prevents someone from making a claim or argument that contradicts what they have previously said or done, or what has been established as true by law. It can be used to prevent the re-litigation of issues or as a defense in a legal case.

Let's say that John tells his neighbor, Jane, that he will not build a fence on the property line between their houses. However, John later changes his mind and builds the fence anyway. If Jane takes John to court over the fence, John cannot argue that he never promised not to build the fence because he is estopped from making that argument. His previous statement to Jane prevents him from making a contradictory claim in court.

Another example of estoppel is promissory estoppel in contract law. This occurs when someone makes a promise or agreement without receiving anything in return (known as consideration). If the other party relies on that promise and suffers harm as a result, the person who made the promise may be estopped from going back on their word.

For instance, 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 later decides not to give the raise. If the employee sues the company for breach of contract, the company may be estopped from arguing that there was no valid contract because the employee relied on the promise of a raise to their detriment.