密封 mìfēng to seal up

怎么记住“密封”: 密 secret + 封 envelope = to seal up.

汉字

Airtight/watertight sealing - 密封的罐子 (mìfēng de guànzi) a hermetically sealed jar; 请密封保存 (qǐng mìfēng bǎocún) keep sealed.
The 密 stresses tightness against air or moisture; contrast plain 封 (fēng, to close/seal an envelope) which just means shut, not airtight.
Common on packaging, food storage, and technical contexts.