“Dont” replaces a noun, a person or a thing. It can have several functions in a sentence, but it always has the function of the object of a preposition. It replaces words that are preceded by “de”, “des”, or “ d’ ”.
“Dont” as a indirect object
Let’s look at this example:
Voici le directeur. Je t’ai parlé de ce directeur. Voici le directeur dont je t’ai parlé. Here is the manager. I told you about this manager. Here is the manager (whom) I told you about.
In the example above we combined two simple sentences into one complex sentence using the relative pronoun “dont”. Notice also that the verb “parler” is followed with the preposition “de”, that’s why we used “dont” instead of other relative pronouns.