This is the 1297th most frequent Russian word.
руководителем
"Руководителем" means "leader" or "manager" in English.
Here, "руководителем" is used as a predicate in the instrumental case to denote someone's role or title.
Мой друг недавно стал руководителем большой компании.
My friend recently became the head of a large company.
In this context, "руководителем" functions as a predicate in the instrumental case, indicating the professional role of the subject.
Моя сестра работает руководителем отдела маркетинга.
My sister works as the manager of the marketing department.
In this example, "руководителем" is used as part of a predicate in the instrumental case to describe the desired state or quality a subject aspires to have.