The ark was a wooden chest overlaid on the inside and outside with gold. It was the most holy physical possession of Israel since it symbolized God's presence (Exodus 25:22; Numbers 7:89; Numbers 10:35–36) with the space between the winged cherubim representing the royal throne from which the Lord reigned (1 Samuel 4:4; 2 Samuel 6:2; Psalm 99:1).1
The ark was to be carried by Levites on special poles and no one was permitted to touch it (Exodus 25:12–13; Exodus 37:3–5; Numbers 4:4–15).
By falling to the ground before the ark, Joshua is bowing down to the Lord.
6 Then Joshua tore his clothes and fell to the earth on his face before the ark of the LORD until the evening, he and the elders of Israel. And they put dust on their heads.