En is a pronoun which replaces a complement introduced by de. For instance,
J'achète des fruits. <-> J'en achète.
It can be a complement of a verb (like in the previous example) or in some cases a noun (I think the noun itself must be direct object of the verb or an attribute linked to the subject by a copula verb).
Il a peur des chiens. <-> Il en a peur.
Je suis malade d'avoir mangé des fruits pas frais. <-> J'en suis malade.
