Essentially, it is a wooden box covered with bronze, designed for easy transportation. Inside, burnt offerings were placed. Compare this instruction with 20:24, where the Lord does not want special altars. This altar, when it was already built in different sanctuaries and later only in the Jerusalem temple, had horns on its four corners. These allowed a person fleeing for safety to grab onto them, thus securing his asylum in the sanctuary (Ex 21:13f; 1 Kgs 1:50; 2:28).