To Stop Something, To Prevent Something
transitive verb, ichidan verb
This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means stop and the verb vocab version of that is to stop something. This particular word is when you're stopping something (i.e. doing the action of stopping it).
とめる
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 need to stop something from moving. Maybe a car... or a door... or a person... whatever you want. To stop this thing, you throw out your toe (と), getting it smashed in the process. Imagine it bleeding and swelling up. How much it hurts! You put your toe out to stop this big thing, what a mistake!
火を止めるボタンはこれです。
This is the button to stop the fire.
いそいで水を止めた。
I rushed to stop the water.
フグは「はっ？」といって、トマトを切る手を止めた。
Fugu said "Huh?," and stopped their hand cutting the tomato.