The ark was a gold-plated wooden chest that held the stone tablets. It was kept in the Most Holy Place in the tabernacle or temple (Exodus 25:10).
Later, they also placed a pot of manna and Aaron's flowering rod in the ark (Hebrews 9:4). The ark was the throne where the invisible Lord sat on earth among his people (see study notes on Exodus 25:22 and Leviticus 16:2).