Sheet erosion refers to the loss of the top layers of nutrient-rich topsoil due to falling rain loosening soil particles and carrying them across the land. Sheet erosion is named so due to the relatively even removal of soil particles, likened to a sheet of topsoil that gets eroded.
Sheet erosion is just one step of the erosion process. It is then often followed by rill and gully erosion. Not all soil particles wind up in water bodies such as rivers and lakes, and they often begin to accumulate on land, which is called rill erosion in small amounts and gully erosion in large amounts.