Refers to office workers, administrative staff, and other non-manual employees. Often used in contrast with 現場労働者 (blue-collar workers).
He works for a large company as a white-collar worker.
現場労働者 refers to blue-collar or manual workers, while 事務系労働者 refers to white-collar or office workers.
Compound of 事務 (office work), 系 (type, system), and 労働者 (worker).