Refers to the historical Japanese Imperial Guards, the military organization that protected the Emperor and the Imperial Palace.
近衛は天皇を守る役目を担っていた。
The Imperial Guards were responsible for protecting the Emperor.
近衛 names the whole Imperial Guards organization, whereas 衛兵 is the general word for an individual sentry or guard on duty, not tied to the Imperial institution.
近衛 refers to the historical Japanese imperial guard as an institution, while 親衛 is a more general term for a ruler's personal guard used across contexts.
Compound of 近 (kon, 'close') and 衛 (ei, 'guard'), referring to guards who are close to the sovereign.