An ember is a little piece of wood or coal in a fire that's dying. Embers are hot and glowing. When the fire is out and the embers are glowing, it’s time to call it a night.
Embers (usually plural) are smoldering pieces of wood or coal — usually very small — that burn brightly as a fire starts to fade. When you see embers, the fire isn’t finished yet, but it’s almost done. If you’re camping and you put out a fire, don’t leave it alone until the embers go out, too. People also use embers as a metaphor for other things that are starting to fade, like the last embers of a dying friendship.