night watch; fire watchman
Refers to a person who keeps watch at night, especially to prevent fires. Historically common in Edo-period towns and castles.
江戸時代には、火の番が夜通し町を見回った。
In the Edo period, fire watchmen patrolled the town all night.
The old fire watchman called out 'Be careful of fire!' while clapping wooden clappers.
Compound of 火 (hi, 'fire') + の (no, possessive particle) + 番 (ban, 'watch, guard'). Literally 'fire watch'.