ancestral tablet; memorial tablet for ancestors
A Buddhist memorial tablet inscribed with the posthumous name of an ancestor, kept on the family altar (仏壇) and venerated.
During Obon, we put our hands together in prayer before the ancestral tablets.
位牌 is the general term for a Buddhist memorial tablet; 先祖の位牌 specifically refers to tablets for ancestors.
仏壇 is the household Buddhist altar where ancestral tablets are typically placed.
Compound of 先祖 (ancestor) and 位牌 (memorial tablet). The term is straightforward and its meaning is transparent from its components.