en_tn_condensed/mat/12/40.md

three days and three nights

Here "day" and "night" mean complete 24-hour periods. Alternate translation: "three complete days" (See: rc://en/ta/man/translate/figs-merism)

the Son of Man

Jesus is speaking about himself. (See: rc://en/ta/man/translate/figs-123person)

in the heart of the earth

This means inside a physical grave. (See: rc://en/ta/man/translate/figs-idiom)