CAPE represents the amount of buoyant energy available to accelerate a parcel vertically, or the amount of work a parcel does on the environment. CAPE is the positive area on a sounding between the parcel's assumed ascent along a moist adiabat and the environmental temperature curve from the level of free convection (LFC) to the equilibrium level (EL). The greater the temperature difference between the warmer parcel and the cooler environment, the greater the CAPE and updraft acceleration to produce strong convection.
CAPE below 0: Stable.
CAPE = 0 to 1000: Marginally unstable.
CAPE = 1000 to 2500: Moderately unstable.
CAPE = 2500 to 3500: Very unstable.
CAPE above 3500-4000: Extremely unstable.