"They shall make an ark of acacia wood. Two cubits and a half shall be its length, a cubit and a half its breadth, and a cubit and a half its height.
The ark was a special box in which reminders of the agreement (covenant) between God and Israel were kept. The tablets on which the commandments were written were in this box, and other things would be added later. Because the ark held these reminders of the covenant, it was sometimes called "the ark of the covenant".
When traditions and rules are made, they have reasons behind them - a purpose. If we forget these reasons, but maintain a high standard for keeping the rules, punishing harshly those who break the rules, people quickly turn away from us. This is a big problem in many churches, who have kept their traditions but have forgotten the reasons for them; and they preserve their traditions proudly but have no clear answer for those who ask why they do so. It is no wonder that religion has a poor reputation among thoughtful people. It is the calling of every follower of God to understand his purpose so that they can speak of it when people ask.
The agreements between God and his people are so important (purpose) that God commanded some very special treatment of the ark. It was to reside in a special place (the sanctuary) and had to be moved without anyone touching it (tradition/rule).