CAPE measures how much energy a rising parcel of air would gain if it got going. It is the area on a sounding between the parcel path and the environment. More area, more potential updraft.
But CAPE is potential, not permission. The cap decides permission. A 4,000 J/kg day with a strong cap produces nothing but a nice sunset and a lot of disappointed people in cars.
Shape matters more than the number. Skinny CAPE spread through a deep layer behaves differently from fat CAPE concentrated low down, which is what drives fast, violent updraft acceleration.
Pair it with shear before you get excited. Energy makes storms tall. Shear makes them organised, and organised is what makes them dangerous.