趁 (chèn) is a preposition that means to 'take advantage of' an opportunity or favorable time to do something. It is normally followed by an adjective or a short phrase explaining the situation that the person is seeking to take advantage of.
Structure
趁 + [Situation / Opportunity]
Examples
趁 年轻，多 去 玩玩 吧。You guys should go have more fun while you're still young.
趁 天气 好，我 要 去 野餐。While the weather is good, I'm going on a picnic.
趁 老板 不 在，我们 抽 一会 儿 烟 吧。When the boss isn't here, we can smoke for a little while.
趁 孩子 睡着 的 时候，我 洗 了 很 多 衣服。While the kids were asleep, I washed a lot of clothes.
小偷 一定 是 趁 我 不注意 的 时候 ，把 我 的 手机 偷走 的。The thief must have stolen my cell phone when I wasn't paying attention.