to seal (e.g. a letter)
to prevent (e.g. movement); to forbid; to block (e.g. speech); to seal off (e.g. a building)
「封じる」 has another reading with different meanings.
手紙を封筒に入れて封じました。
I put the letter in an envelope and sealed it.
怒りを心に封じて我慢した。
I suppressed my anger in my heart and endured it.
手紙を封じないまま机の上に置いてあった。
The letter had been left on the desk without being sealed.