This lotion will help keep the moisture in your skin.
This may look like an adjective, but it's actually the noun version of the kanji watered. Watered things, like plants, hold a lot of moisture in them.
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 潤う, so you should know this one too.