Soil is actually a mix of Water, Air, Organic Matter and Minerals. Roots grow in the space between particles- or air pockets.
When heavy pressure from water or machines presses down on soil, it packs down minerals and organic matter and reduces the air in the soil. This process is called compaction. One of the most important ways to build a good soil is to make sure it has plenty of air in it. Tilling is bad for soil (will explain why in another episode), but a compacted soil needs to be loosened up so air can return to the root zone.