Most of the time when you pass your mouse over one of the indivdual 2D images on the screen you will be shown its name above your characters' portraits, like tree or rock. Occasionally the name will be capitalized or absent, indicating that this feature has more than what meets the eye. Click on the image to receive a prize. You may need to use your Spacebar instead.
Sometimes you'll find a trapped Chest. Sometimes you'll get experience points. Sometimes you'll be given an Item and sometimes an Item will be taken away.
Some of these features will only work if the active character has at least a certain level of a particular Skill. Give all your characters a chance at it, perhaps coming back later with a different or more experienced team. Regardless of whether you get your prize or not, a single white square will appear on your map to mark the location of the feature.