A foghorn is a warning signal that makes a loud noise when it's very foggy. The sound of a foghorn is a warning to sailors and ship captains.
On foggy days near the coast, you might hear the repeated low, loud sound of a foghorn. Its warning noise lets those at sea know when they're approaching a rocky coastline or other obstacles they can't see in the fog. The earliest foghorns were gongs or bells that had to be manually hit, or in some cases cannons that were fired repeatedly as long as the fog lasted.