This refers to dead people. Alternate translation: "Those who are dead" or "The spirits of the dead" (See: [[rc://en/ta/man/translate/figs-nominaladj]])
They tremble because they are afraid of God. This can be stated clearly. Alternate translation: "tremble in fear" or "tremble in fear of God" (See: [[rc://en/ta/man/translate/figs-explicit]])