Pure
Clean
い adjective
This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of pure? It's pure.
きよい
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: "In order to stay pure and clean, you shouldn't ever do anything dirty or bad. That is the key, yo (きよ)."
彼女は清い心を持っており、人々をとても思いやります。
She has a pure heart and is very considerate of others.