Don’t let this English-teacher term worry you. All it means is that you refer to something in words different than the ones you just used:
1. George Washington, Father of his Country, built Mount Vernon.
2. Paris, the City of Light, is almost deserted in August, when residents go on their vacations.
3. Hawaii, the Big Island, has an active volcano.
4. Carolyn, my friend from high school, lives in Montana.
I’ve underlined the appositives in those sentences. What do you notice about them? They are all set off by commas on both sides.