The authors of Divinity: Original Sin have prepared a whole lot of secrets and hidden treasures on the map. The first ones often connect with the second, but in the game guide, secrets marked on the map with numbers are "more important" and more valuable. You can get the maps to these secrets from Geremo, in the Cyseal's market, or from Teller of Secrets in one of the portals in The End of Times, after completing the "Healing the Mount" quest (Luculla Forest).
Most of the secrets contain buried chests, but you may sometimes encounter something else, like for example skill books. After buying and reading a map, info about the secret appears in the "Secrets" tab and the localization is marked on player's map (an icon of a small chest). When you mouse over the icon, a short description of the secret will appear.
Usually, you will have to dig in a pile of dirt to get to the chest (the content of a chest is always random, but the items are of similar value). In order to do that, you will need a shovel. You will find one in the Cyseal's graveyard. If you happen to lose it, you can buy one from a merchant, but it's a rather rare item. After arriving at the location of the treasure, look for a small pile of dirt and start digging by clicking on it (the shovel has to be in the equipment of the person digging, not in the companion's).
Important! To locate a treasure, you don't have to buy a map. However, to notice it, you will need a lot of Perception (preferably more than 7). Thus, if you intend to look for treasures, develop the skills needed in one of your party members and equip him with items that increase perception value. You can also use potions that enhance perception, if you still do not see the treasure.
Some treasures are not marked as secrets, but have similarly valuable content and may be difficult to open. On your map, you can see treasures that require more effort than jus opening a chest.