This is the 3548th most frequent Chinese word.
沉
"沉" means "to sink," "deep," or "heavy."
沉 here is used to describe a physical characteristic of heaviness, applied to luggage.
沉重的行李让他走得很吃力。
The heavy luggage made his walking quite strenuous.
沉 here is used to describe a mental or emotional state of being calm or quiet.
他的表情沉静，似乎在思考什么问题。
His expression is quiet, as if contemplating an issue.
沉 here is used metaphorically to describe the action of the sun sinking, in relation to its movement below the horizon.