To become warm; to warm up (oneself or a place). Used when a body or space gets warmer, e.g. after getting into a bath or when a room heats up. Colloquially often said as 'あったまる'.
寒かったので湯船に入って体を温まらせた。
It was cold, so I got in the tub to warm my body.
部屋がストーブでだんだん温まってきた。
The room gradually warmed up thanks to the heater.
朝晩は冷えるから、しっかり温まってから寝てね。
It gets chilly in the morning and evening, so make sure you warm up well before sleeping.