• 封鎖ふうさ — blockade; shutting down or sealing off an area
• 封書ふうしょ — sealed letter; letter in an envelope
• 封印ふういん — a seal placed on something; to seal up
• 密封みっぷう — airtight seal; sealing something completely
• 開封かいふう — opening a sealed letter or package
• 封ふうをする — to seal; to put a seal on an envelope or container

With the measuring radical 寸 on the right, 封 originally pictured marking off land and closing it. Think of driving a stake into the ground at the border — once it’s marked and measured, the space is sealed and no one enters without permission.