A smoldering fire is one that's still burning, but not very strongly. You might see some small flames, or even just glow from embers.
When we say something is smoldering, we usually mean it's still happening or feeling strong inside, but maybe not as much on the outside. For example, if someone is really upset about a situation, but they're keeping their cool and not showing it, you might say their anger is smoldering. It's like a fire that's burning quietly beneath the surface. This can also apply to other feelings or emotions, like resentment or frustration.
To burn with no flame and little smoke.
(sometimes figurative) The act by which something smoulders; residual heat.
Example: the smoulderings of the Thirty Years War
