Relative pronouns are those that introduce a relative clause (a clause that modifies a noun and that does not express a complete thought) and that refers to nouns mentioned previously. There are few relative pronouns, some of them are <em>who, whom, whoever, whomever, whose, which </em>and<em> that.</em>
In the sentence, then, the relative pronoun is "whom" and it relates its clause ("for whom I work") to the noun "manager."