The car stopped at the red signal.
I was so surprised I thought my heart leapt into my mouth.
I couldn’t stop on the ice.
This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb.
This is an intransitive verb. That means it does not take a direct object. Something comes to a stop. No direct object here. That's why this verb means to stop.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
You come to a stop sign and decide to stop because the sign has a picture of a toe (と) on it. This toe looks eerily like a toe your Ma (ま) has too... and your Ma always taught you to stop and look both ways before crossing her toes.