What does each polygon represent?

Each polygon represents a soil map unit.

When working with SSURGO soils data (what is SSURGO data?), each map unit represents an area dominated by one to three kinds of soil. The map unit is named for the dominant soil or soils.

The boundaries of soil map units do not necessarily represent an abrupt change of soil type. Usually, the transition occurs over many feet. Map units also do not represent homogenous soil bodies. Every soil body contains inclusions (small bodies of other soils). It is impossible to represent the entire variety of the Earth's soils so soils maps smooth over many of the variations.