The Ark of the Covenant was a box in the Holy of Holies of the wilderness Tabernacle. This space was only accessed by the high priest once a year on behalf of Himself and the people.
The Ark contained the copies of the commandments which were seen as the relational agreement between God and man. There were two other items in the ark, an almond branch that budded, recognizing the right of the Aaronic priesthood, and a golden pot of manna as a memorial of God's provision for His people during their wilderness journey.
The Ark has also been referenced in Scripture as God's footstool and His presence was said to have dwelt between the two Cherubim. These were a part of the mercy seat that covered the box as types of guardians of His presence.