Soil bearing capacity
How much load a given soil can support per unit area without excessive settlement. It determines how wide a footing must be.
What does soil bearing capacity mean?
Bearing capacity varies enormously between dense gravel, stiff clay, and loose fill, and residential codes provide presumptive values by soil class that let builders size footings without testing. Where soils are questionable, expansive, or the loads are unusual, a geotechnical investigation replaces the presumptive table. Bearing on undisturbed soil is the assumption behind all of it.
In practice
An inspector rejecting a footing trench because the bottom is loose or soft is enforcing this concept directly.