Soil is the base substrate for most plants. It's made from organic and mineral materials and should contains life to be healthy. Good soil hosts hundreds or thousands of different organisms. From mushrooms to worms, all these life contribute to nourishing and oxygenating the soil.
Outside, each soil contains a different amount and variety of texture and life. Adding to that the local weather, we call it the terroir.
Inside, soil will partly determine the quality of the crop (the other part depends on the genetic and type of light). Usually, organic and alive soil give best results, allowing the plant to show its whole potency.
Be careful when buying soil in supermarkets or random plant shops, they are often low quality, and are sometimes not clean. The most common problems is that black gnats are already living in there or unwanted mushrooms have already developed.
Best soil is usually found in grow-shops. There are mainly 2 kinds: the All Mix and the Light Mix.
All mix soil contains in it a fair amount of nutrients, which means there will be no need to add nutrients during 3 or 4 weeks. Of course it is possible to give additional nutrients to maximise the crop, but not completely necessary.
Light mix soil contains very little amount of nutrients. After 2 weeks, it becomes imperative to give nutrients otherwise plants will rapidly ask for food and stop growing (or blooming).
A different approach is to use coco fiber base substrate. This substrate is particular because it allows roots to breath much more than soil. At the same time, it gets dehydrated very fast, it is common to have to water everyday (or even more) when plants are well developed. Also, the supply of nutrients is more consistent because it is drained more easily. This substrate is more related to hydroponic technique because of its features.