Nous avons discuté de la situation. => Nous en avons discuté.
En is a pronoun that replaces
de la situation. It is not a preposition.
In English it would be conveyed by:
We have discussed about it.
Usually in English one can omit
about it. In French, it isn't so.
Il en avait compris l'importance => He had realized the importance (of it).
As a side remark, note that
en as preposition is not always translated by
En allant à l’université, j’ai rencontré une belle fille = While going to college I met a beautiful girl.
Aller en Amérique = Go to America.