小老婆 is an informal, somewhat derogatory term for a concubine — a woman in a secondary marital relationship to a man who already has a primary wife. The term literally means 'little wife' and carries strong patriarchal and historical connotations. In modern Chinese, it is considered offensive and is rarely used in polite conversation; you are more likely to encounter it in historical fiction, legal contexts (referring to pre-1949 polygamy), or as a slur. The neutral modern term is 妾 (qiè) or 情妇 (mistress).