https://dictionary.cambridge.org/zhs/%E8%AF%8D%E5%85%B8/%E8%8B%B1%E8%AF%AD-%E6%B1%89%E8%AF%AD-%E7%AE%80%E4%BD%93/drain
If you drain something, you remove the liquid from it, usually by pouring it away or allowing it to flow away, and if something drains, liquid flows away or out of it.