Here the word "sign" refers to something that communicates a special warning to those who see it. Ezekiel speaks of himself and his actions as being this warning. Alternate translation: "I am a warning to you" (See: [[rc://en/ta/man/translate/figs-metaphor]])
The word "them" refers to the people living in Jerusalem. This can be stated in active form. Alternate translation: "so others will do to them" (See: [[rc://en/ta/man/translate/figs-activepassive]])