Buoyancy is the upward or downward acceleration on an air parcel produced by the density difference between the parcel and the air surrounding it at the same level. When the parcel is warmer, and therefore less dense, than the environment, buoyancy is positive and the parcel accelerates upward. When it is cooler and denser, buoyancy is negative and the parcel accelerates downward.
The physics
The buoyancy force per unit mass is proportional to the fractional difference in temperature between the parcel and the environment at the same pressure. A parcel one degree Celsius warmer than its surroundings feels a small upward acceleration. Ten degrees warmer, and the acceleration is roughly ten times larger. Latent heat release from condensation makes rising saturated parcels warmer than the dry environment, which is the reason storm updrafts can reach several tens of meters per second.
Water content matters as much as temperature. Suspended droplets, raindrops, and ice add mass to the parcel without adding volume, which reduces its effective density and cuts into positive buoyancy. This water loading is why the strongest observed updrafts still fall short of the values that pure temperature-only buoyancy would predict.
How forecasters use it
Buoyancy is not usually measured directly. It is inferred from the Skew-T sounding by comparing the parcel's temperature curve with the environmental temperature curve at each level. The area between the two curves, integrated over height, is CAPE for positive buoyancy above the level of free convection and CIN for negative buoyancy below it.
Downdraft CAPE, or DCAPE, applies the same idea in reverse for a parcel descending from mid-levels. Both diagnostics assume undiluted parcel motion, so they describe the ceiling of what buoyancy can do rather than what a real storm updraft or downdraft actually achieves.
Where it misleads
Buoyancy calculated from parcel theory is an upper bound. Entrainment mixes drier environmental air into the rising column, condensate loading adds weight, and turbulent drag along the edges of the updraft all reduce actual accelerations below the pure parcel value. Observed updraft speeds are commonly a fraction of the value CAPE alone would suggest.
Buoyancy also depends on which parcel is being lifted. A surface parcel, a mixed-layer parcel, and a most-unstable parcel from the same profile produce three different buoyancy curves and three different CAPE values. The right parcel is the one that describes the air the storm is actually ingesting, and that choice depends on the boundary layer state at initiation time.