Topsoil is the upper, outermost layer of soil, usually the top 5–10 inches.
This top layer is where the majority of plants lay the most of their roots, as they are able to absorb nutrients that allow them to grow healthily, due to the high concentration of organic matter and important minerals.
If the soil in your garden is very shallow, or you are making new garden beds, adding or replacing a lawn, or putting in raised beds and need to add new soil, topsoil is what you need.