Source: DPI NSW

Soil is composed of sand, silt, clay and organic matter. The minute particles of sand and silt are bound by clay and organic matter into aggregates. These are the crumbs or lumps soil breaks into when you dig it. The technical term for them is peds.
The arrangement of aggregates gives soil its structure.

Good soil structure has adequate spaces (pores) between aggregates to allow water and air to enter the soil and drain easily, while holding enough moisture to maintain plant growth. Poor soil structure has few aggregates and few pores between soil particles.

When looking at your soil structure, all you have to basically think is how much air can move through this soil. Then look at how the soil breaks down when it’s wet and see how this airflow might be reduced.