What is the High Trees principle?

In Combe v. Combe [1951] 2 K.B. 215, Lord Denning at page 220 restated the High Trees principle in the following words: "The principle, as I understand it, is that, where one party has, by his words or conduct, made to the other a promise or assurance which was intended to affect the legal relations between them and to be acted on accordingly, then, once the other party has taken him at his word and acted on it, the one who gave the promise or assurance cannot afterwards be allowed to revert to the previous legal relations as if no such promise or assurance had been made by him, but he must accept their legal relations subject to the qualification which he himself has so introduced, even though it is not supported in point of law by any consideration but only by his word."