What is a core worker in business?

Could you please elaborate on the concept of a CORE worker in the business realm? Are they individuals who hold essential roles within an organization, playing pivotal parts in its day-to-day operations and long-term success? Are there specific qualities or skills that define a core worker, and how do they contribute to the overall functioning and growth of a company? Additionally, how does the role of a core worker differ from other employees within the same organization?