If you are serious about topsoil or why exactly your yard is in need of top soil, you might want to watch this video and learn more about it. Topsoil is basically the top layer of soil; if you have ever been to a river bank, you have probably seen that the top layer of soil of the many layers is darker than the rest of the soil, and relatively lush as well. In soil classification systems, it is known as the A horizon.
This soil is basically the top two to seven inches of soil, containing the highest concentration of organic matter and microorganisms. Topsoil is basically made up of nitrogen and carbon, as well as the life forms that live in it. If the soil is fertile, it also contains nutrients that can help plants grow within it.
The most important function of topsoil is to provide nutrients to plants that are growing within it. The diverse community of microorganisms that grows within the topsoil also plays a part in nurturing the growth of the plants, which creates an ecosystem that is conducive to the growth of life in the soil.
Essentially, there is quite a bit more to topsoil then simply having a nice layer of soil on top to look at.