To Soak, To Steep, To Dip
transitive verb, godan 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 immersed, so the verb version is what happens when you immerse something in a liquid. It's to soak or to dip.
ひたす
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You already learned this reading when you learned 浸る, so you should know this one too.
どうして野菜を水に浸しているの？
Why are you soaking the vegetables in water?