To steam (something) — to cook or heat with steam (transitive).
蒸し器で野菜を蒸すと栄養が逃げにくい。
If you steam vegetables in a steamer, their nutrients are less likely to be lost.
中華まんを蒸すために蒸し器を用意した。
I prepared a steamer to steam the steamed buns.
濡れたタオルを蒸して温める。
Steam a wet towel to warm it up.
To be hot and humid; to feel sultry or muggy (intransitive).
今日は蒸すね。エアコンをつけようか。
It's muggy today. Shall I turn on the air conditioner?
雨が降ったせいで部屋が蒸している。
Because of the rain, the room is humid and stuffy.