I do not have an agronomy PhD, but here is my naive understanding: Soil compaction is a change to the structure of soil. The soil is squeezed by the load of the tracks and large wheels of heavy agricultural machinery repeatedly running over it. The soil particles are bonded together. A result is that water is poorly absorbed, and not held in the soil as well. Plants roots have difficulty penetrating compacted soil. Water runs off and the risk of flooding increases.

It is primarily an agricultural problem. Amelioration by one-time plowing might not be completely effective if the turned soil clumps remain compacted, especially in soils with high clay content. Another amelioration strategy might be to seasonally change large vehicle traffic patterns to avoid compressing the soil year after year. (AI was not consulted for this content)

Dr George McIlvaine thank you for your contribution to the discussion

Soil compaction is the practice of applying mechanical compactive effort to densify a soil by reducing the void space between soil particles. Compaction occurs when particles are pressed together to reduce the space between them. Highly compacted soils contain very few spaces resulting in soil with higher unit weight. The most common causes of compaction on farms are animals and machinery, mostly tractors and heavy cultivation and harvesting equipment. The degree of compaction depends on the force compressing the soil, the contact area with the soil, the strength in the soil and the type of soil. The result is soil compaction and it concerns the negative effect of driving heavy machinery on soil that is used for growing plants. Soil compaction is characterized by increased density of the soil, reduced air volume and a reduced ability to drain off surplus water. Amend your soil by adding organic matter, like compost. Decomposing organic matter helps build soil aggregates. Aerate the soil, which removes small plugs of soil to alleviate compaction and prevent thatch accumulation. Enrich the soil with organic matter. Organic matter in soils improves soil structure and aggregation. This forms a greater resistance to the force from compaction causing agents. Organic matter in soils can be increased by adding green manure, or animal manure to the soil. Compaction decreases the likelihood of settlement after a building, roadway, runway or parking lot is constructed which could result in premature pavement failure, costly maintenance or repairs. Soil compaction is a crucial part of the construction process as it provides a strong working platform. The only way to eliminate soil compaction is not to drive on the soil. Practices such as no-till-, and conservation tillage agriculture can greatly reduce the amount of stress placed on the soil. There are also many technical solutions that can reduce soil compaction and improve yields.