Denarii are an important currency in the game. They are used for a variety of different purposes:

You will passively generate Denarii from your houses. The higher the level of the house, the more Denarii it will generate per day. You do not need to collect the Denarii from the houses, rather, it accumulates in your suburb's forum. Since there is a maximum amount of Denarii that your forum can hold based on its level, you will need to be sure to collect this Denarii from the forum frequently and to upgrade the forum so that you can hold more.

You can also get Denarii from selling your extra goods at the wharves in your city. If a wharf is full, you can either open another wharf, upgrade the wharf, or export some goods from your city to make room to sell more.