The factor of safety compares what a system can resist against what is trying to fail it. In slope stability it is the ratio of shear resistance to driving shear stress on the potential failure plane — for a dry cohesionless infinite slope it collapses to the elegant FoS = tan(φ)/tan(β), so the slope sits at limiting equilibrium (FoS = 1) exactly when its angle reaches the friction angle, the angle of repose. In foundation work the same idea divides capacity: the gross allowable bearing pressure is the ultimate capacity over the factor, q_all = q_ult/FS, with FS = 3 the typical choice for shallow footings.
A value above one means theoretically stable, but nobody designs to 1.0. Long-term slope design typically demands 1.3 to 1.5 to absorb the uncertainty in strength parameters, pore pressures, and loading — and the margin exists because conditions change: with steady seepage parallel to a slope and the water table at the surface, the friction term drops to the buoyant unit weight while the driving stress keeps the full saturated weight, cutting a slope’s factor of safety from 1.86 dry to 1.17 wet in the worked example. Values hovering near unity are a signal for detailed investigation, not a passing grade.