Something gets eroded if it is being worn away. Mountains slowly erode over time and the rock they are made from turns to dirt. Erosion is only a problem if it happens too fast. When people cut down a lot of trees and plants all at the same time, there is nothing to hold the dirt together so it either gets blown away by the wind or washed away by rain. Once the soil has eroded away there is nothing to grow plants in for food, and the land becomes useless. The picture is of some bad erosion around a creek.