When a fire is smoking but not in flame, it is smoldering. If you're silently angry, your contained rage is smoldering, too.
Just like a fire that doesn't go out completely, this word is used for emotions that also refuse to die out — they just keep going, though under the surface. If you keep a lid on your feelings, but are mad at someone, your feelings are smoldering. People often talk about smoldering feelings of love or lust: in romance novels and romantic comedies, there will be smoldering feelings and smoldering looks. This is a word for emotional heat that keeps burning.