Topsoil is essentially organic matter. Its is the upper layer of soil on our earths crust. Our earths crust is made up of five layers, the second layer being topsoil.
During the excavation of most building and gardening projects the outer layers of soil are usually removed or in some part destroyed leaving you with silt, sand or clay within your garden. This is when topsoil is usually required. Topsoil is typically high in nutrients and is used for flower beds, lawns or where soil in non-existent. It is rich in nutrients and has the right acidity and microorganisms for plants to feed off, all the while providing drainage and locking in moisture around the roots of plants.
In order to be considered as premium, Topsoil is graded by its physical structure, texture, composition, acidity and contaminants. The BSI 3882:2015 provide the certified standard for premium quality topsoil, whereby the soil itself has to meet a stringent set of requirements and be free of contaminates. Its is very important that topsoil does not contain any contaminants as not only can this have a detrimental impact to your garden but wilfully and knowingly contaminating land get you into serious trouble with the local authorities and under the Environmental protection Act 1990 (EPA).
If you want to know more about the grading of Topsoil you can visit the BSI website for more info.