What is the meaning of the rule of estoppel?

The above statement employs the phraseology of the original statement of the rule of estoppel contained in the judgment of Lord Denman, C.J. in Pickard v. Sears (1837) 6 Ad & El 469, 112 ER 179, at 181: “But the rule of law is clear, that, where one by his words or conduct wilfully causes another to believe the existence of a certain state of things, and induces him to act on that belief, so as to alter his own previous position, the former is precluded from averring against the latter a different state of things as existing at the same time.”