Soil is a composite of organic matter, micro-organisms, minerals, air and other inorganic materials. Soil is often viewed as the boring part of gardening and while it will never be as glamorous or as interesting as when choosing your plants. New gardeners are urged to put money and effort into improving their soil. Few however appreciate this wisdom until they see that their plants are not doing so well and need more food and water.
Typically soil is 90% mineral residue and 10% organic material and sustains a community of insects and microorganisms. The reason for adding additional organic matter to your soil is to provide food for the beneficial microorganisms that release nutrients into the soil as they decompose the organic matter. Earthworms and other soil dwelling insects aerate the soil as they move through it and contribute still more organic matter with their waste and decomposition. This makes for what is called. Fortunately we don't need to know everything about soil to grow our plants and in most container gardening you won't be using garden soil. When you remove topsoil and put it into a container you lose many of those natural support systems and are left with material that is too dense and heavy for plants to grow in well.
So for container (planter) gardening you try to simulate the best conditions topsoil can offer, adequate air, beneficial microorganisms, organic matter and nutrients. Trying to do this in a form that won't compact and be too dense.