the.com/public address system

means A system of microphones, amplifiers, and loudspeakers used to broadcast announcements or sound to a crowd across a large space.

from A straightforward compound: 'public' (open to all, from Latin publicus) plus 'address' in its sense of speaking to someone, plus 'system' for the wired network of equipment. The term and the technology grew up together in the early 20th century, once amplification made it possible for one voice to reach thousands — the phrase is simply descriptive, no hidden story, though the abbreviation 'PA' has since taken on a life of its own as the generic name for any speaker rig, from airports to garage bands.