This word consists of kanji with hiragana attached. Because the hiragana ends in an う sound, you know this word is a verb. More specifically, this verb ends in まる, and most verbs with this ending are intransitive, meaning the action or change happens on its own.

The kanji 温 means warm. So this verb means to get warm or to warm up. To help you distinguish this from 温める, you could think of a circle (まる) rolling down a hill all by itself, and getting warmer and warmer as it picks up speed.