Soil

If you don’t really know what good topsoil is:
It will be brown, black or even reddish in color.
It smells like good fresh dirt.
It breaks apart very easily when moist.
Is porous, yet holds moisture-it takes a small hole filled with water several seconds to drain out.
It will have several large earthworms when you turn over a small shovel full.
You can see organic matter such as leaves, plant stems etc. that are apparently rotting and/or breaking down.

If you’re lucky enough to have anything close to topsoil, you’re in good shape. If not, you can develop, and/or improve it.
The roots of most vegetable plants are within a foot of the surface. I’ve read that lettuce roots will go to 4 feet deep, tomatoes to 10 feet deep, and so on. All that may be, but the greatest mass of root structure will be in the top 12 inches of soil.
As my entire lot was nothing but hard clay that would grow nothing similar to a decent plant, and having tried and failed at every means of turning that clay into something like soil, I gave up that enterprise. I built large boxes on top of the clay, about 2 feet high, and filled them with half each, bags of store bought topsoil and compost. That can get a little expensive if you live in the city and don’t have access to free barnyard manure or river bottom soil that you can import to your boxes. I found a nursery that gave me a great price on the topsoil and compost for the quantity I needed.