Peaceful: Sims with this trait don't like to fight with other Sims. Their angry or sad moods will have to be super powerful for your sims to go feel this way. These Sims will also not really be sad or angry. The fine emotion is usually overpowering. Their mood will not really fluctuate as much. Stoic: Sims with this trait will usually just be fine.
They will usually have moods that lean more on the serious side, like focused and confident, and rarely energized or happy, and even more rare for them to be in a playful mood. Serious: Sims with this trait can hardly get playful.