Archaic term for a person who guards or watches over something. In modern Japanese, 守護者 or ガードマン are more common.
In old tales, the castle guard kept watch all night.
Compound of 守り (mamori, 'protection') + 人 (hito, 'person'), with rendaku voicing of ひと to びと. The exact historical period of coinage is uncertain, but it follows a classical pattern for agentive nouns.