Problem understanding this – Please help

Promissory estoppel is where it prevents a person from going back on his promise to accept lesser amount.

In Central London Property Trust v High Trees House:
Centeral London cannot claim $5000 which is lost during the war period from 1941 to 1945 because it gave its promise to accept lesser amount of rent from High Trees House.

The doctrine of promissory estoppel prevents Central London to claim the rest of the rent which they promised to waive by accepting less amount of rent.

According to the doctrine of promissory estoppel if somebody accepts the lesser amount by giving his promise to the other party acted upon by paying lesser amount then the first party cannot claim the full amount afterward (once they have already promised).

Is this correct?