Refers to the person who earns the primary income for a household. Similar to 稼ぎ手, but 稼ぎ主 emphasizes the main earner role.
See also: 稼ぎ手 (かせぎて)
He is the breadwinner of the family.
When the breadwinner loses their job, the household finances become strained.
Compound of 稼ぎ (kasegi, 'earning') and 主 (nushi, 'master' or 'head'). The exact historical derivation is uncertain, but the word transparently means 'earning master'.