Refers to a male domestic servant, typically in a historical or literary context. Not commonly used in modern everyday Japanese; 使用人 or お手伝いさん are more common for general domestic help.
彼は若い頃、大きな屋敷で家僕として働いていた。
When he was young, he worked as a houseboy in a large mansion.
時代劇では、家僕が主人
In period dramas, scenes of a manservant faithfully serving his master often appear.
Compound of 家 (house) and 僕 (servant). The exact historical derivation is uncertain, but the kanji combination directly suggests a domestic servant.