Equitable Estoppel in layman's term is being honored for one's word or actions; your word is your bond.
In Planters Development Bank v. Spouses Lopez (720 Phil. 426, 441-442 (2013)), the Supreme Court expounded on the principle of estoppel as follows:
Section 2, Rule 131 of the Rules of Court provides that whenever a party has, by his own declaration, act, or omission, intentionally and deliberately led another to believe that a particular thing is true, and to act upon such belief, he cannot, in any litigation arising out of such declaration, act or omission, be permitted to falsify it.
The concurrence of the following requisites is necessary for the principle of equitable estoppel to apply: (a) conduct amounting to false representation or concealment of material facts or at least calculated to convey the impression that the facts are otherwise than, and inconsistent with, those which the party subsequently attempts to assert; (b) intent, or at least expectation that this conduct shall be acted upon, or at least influenced by the other party; and (c) knowledge, actual or constructive, of the actual facts.
Inaction or silence may under some circumstances amount to a misrepresentation, so as to raise an equitable estoppel. When the silence is of such a character and under such circumstances that it would become a fraud on the other party to permit the party who has kept silent to deny what his silence has induced the other to believe and act on, it will operate as an estoppel. This doctrine rests on the principle that if one maintains silence, when in conscience he ought to speak, equity will debar him from speaking when in conscience he ought to remain silent (De La Salle Araneta University v. Bernardo, February 13, 2017, G.R. No. 190809).