"Foraging" lets you see natural objects on the ground and collect them by right clicking it and selecting "Pick". As you raise Perception or Exploration you'll be able to see more of these objects and see them more often, e.g. rustroot, curiosities like flotsam, food like blueberries + chantrelles, etc.
To forage, run around in a forest until you see some items you can collect. If you don't have the Foraging skill you can't see any of these items. They're usually easy enough to differentiate from background terrain.
http://ringofbrodgar.com/wiki/Foraging