en_tn/rev/07/03.md

put a seal on the foreheads

The word "seal" here refers to a mark. This mark shows that the people belong to God and that he will protect them. Alternate translation: "put a mark on the foreheads" (See: rc://en/ta/man/jit/figs-metonymy)

foreheads

The forehead is the top of the face, above the eyes.