上辈子 has two connected but distinct meanings. One is the literal 'previous life' in the Buddhist reincarnation sense — your existence before this one, which people invoke to explain luck, karma, or compatibility. The other is 'previous generations' or 'forefathers' in a lineage sense, referring to ancestors a few generations back (as opposed to 祖宗 for ancient ancestors). The first meaning is more common in everyday conversation and idioms, while the second appears in historical or family contexts.