Ahsanallahu ilaykum What is “التابوت" in Surah Al Baqarah

Imam al-Sa‘dī said: Then their prophet also mentioned to them a tangible, visible sign: the coming of the Ark, which they had lost for a long time. In that Ark was a tranquility that brought peace to their hearts and reassurance to their minds, and within it were remnants left behind by the family of Mūsā and the family of Hārūn. The angels brought it, carrying it while they looked on with their own eyes.